Imaging optical system

ABSTRACT

The disclosure generally relates to imaging optical systems that include a plurality of mirrors, which image an object field lying in an object plane in an image field lying in an image plane, where at least one of the mirrors has a through-hole for imaging light to pass through. The disclosure also generally relates to projection exposure installations that include such imaging optical systems, methods of using such projection exposure installations, and components made by such methods.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority under 35 U.S.C. §119(e)(1) to U.S. Ser.No. 60/885,261, filed Jan. 17, 2007. This application also claimspriority under 35 U.S.C. §119 to German patent application serial number10 2007 003 305.4, filed Jan. 17, 2007. These applications areincorporated herein by reference.

FIELD

The disclosure generally relates to imaging optical systems that includea plurality of mirrors, which image an object field lying in an objectplane in an image field lying in an image plane, where at least one ofthe mirrors has a through-hole for imaging light to pass through. Thedisclosure also generally relates to projection exposure installationsthat include such imaging optical systems, methods of using suchprojection exposure installations, and components made by such methods.

BACKGROUND

Imaging optical systems are known as projection optical systems as acomponent of projection exposure installations for microlithography.Imaging optical systems are also known in conjunction with microscopelenses for inspecting masks or wafers.

SUMMARY

In one aspect, the disclosure features an imaging optical system thatincludes a plurality of mirrors configured to image an object fieldlying in an object plane in an image field lying in an image plane. Atleast one of the mirrors has a through-hole configured so that imaginglight can pass therethrough. A reflection surface of at least one mirroris in the form of a free-form surface which cannot be described by arotationally symmetrical function.

In another aspect, the disclosure provides a projection exposureinstallation that includes an imaging optical system as described in thepreceding paragraph, and a lens system configured to direct illuminationlight to the object field of the imaging optical system. The projectionexposure installation is a projection exposure installation formicrolithography.

In a further aspect, the disclosure provides a method that includesusing the projection exposure installation described in the precedingparagraph to produce a microstructure on a wafer.

In some embodiments, the disclosure provides an imaging optical systemwherein a reflection surface of at least one mirror is in the form of afree-form surface which cannot be described by a rotationallysymmetrical function.

It has been recognized that using free-form surfaces instead ofreflection surfaces having a rotationally symmetrical axis provides anew level of design freedom which results in imaging optical systemswith combinations of properties which are not possible with rotationallysymmetrical reflection surfaces. The free-form surface cannot bedescribed by a function which is rotationally symmetrical about a markedaxis representing a normal axis to a surface portion of the opticalsurface. The free-form surface thus cannot be defined in particular by aconic section-aspheric equation. Aspheres of this conic type deviatefrom spherical symmetry but can be described, however, by a rotationallysymmetrical function, namely a function which is dependent on only oneparameter, namely the distance to an optical axis, whereas the free-formsurfaces require at least two parameters which are independent of oneanother to describe the surface. Conic section-aspheric surfaces aretherefore not free-form surfaces. The shape of a boundary of theoptically effective surface is not significant. Optically effectivesurfaces which are not bounded in a rotationally symmetrical manner areknown. Optically effective surfaces of this type can nevertheless bedescribed by a rotationally symmetrical function, anon-rotationally-symmetrically bounded portion of this optical surfacebeing used. The free-form surface may be a static free-form surface. Theterm “static free-form surface” refers to a free-form surface, the shapeof which is not actively modified during use of projection in theprojection optical system. A static free-form surface can of course bedisplaced for adjustment purposes. The free-form surface can, inparticular, be constructed on the basis of a planar reference surface orbasic shape, a concave reference surface or a convex reference surface.In particular, at least one free-form surface may be used which isconstructed on the basis of a curved reference surface. In this case, areference surface with a vertex curvature which is constant over theentire reference surface can be used. A conic section-asphere may alsobe used as a reference surface. In conventional imaging optical systemsincluding a through-hole, which are known as pupil-obscured systems, theuse of this type of free-form surfaces can enable compact imagingoptical systems with a low level of imaging errors to be achieved and,in particular, a high light throughput to be produced. According to thenumber of mirrors in the imaging optical system, a single mirror, or aplurality of mirrors, or all of the mirrors of the imaging opticalsystem may be in the form of free-form surfaces. The free-form surfacescan have a maximum deviation from a rotationally symmetrical surface,which is best-fitted on the free-form surface and which does notnecessarily match a designed reference surface, of at least the value ofthe wavelength of the imaging light. The deviation of, in particular, atleast the value of a wavelength of the imaging light is, in practice,always markedly greater than the manufacturing tolerances duringproduction of optical components for microlithography which, in absoluteterms, are conventionally 0.1 nm and, in relative terms, areconventionally 1/50 or 1/100 of the wavelength of the illumination lightused. In the case of illumination with EUV wavelengths, the deviation isat least several tens of nm, for example 50 nm. Larger deviations, forexample 100 nm, 500 nm or 1,000 nm or even larger deviations are alsopossible. When using systems with imaging light of higher wavelengths,even greater deviations are possible. A free-form surface may beprovided, for example, by a biconical surface, i.e. an optical surfacewith two different basic curves and two different conical constants intwo directions perpendicular to one another, by a toric surface or ananamorphic and, at the same time, in particular, aspheric surface. Acylindrical surface therefore also represents a free-form surface ofthis type. The free-form surfaces may be mirror symmetrical to one ormore planes of symmetry. The free-form surface can be a surface withn-fold symmetry, n being a whole number and greater than or equal to 1.The free-form surface may also have no axis of symmetry and no plane ofsymmetry at all.

Different ways of describing optical surfaces, in particular anamorphicsurfaces, are described in U.S. Pat. No. 6,000,798, for example, whichis hereby incorporated by reference. Analytical formulae for describingnon-rotationally-symmetrical surfaces, in particular anamorphicaspherical surfaces, toric surfaces or biconical aspherical surfaces,are also described in WO 01/88597, which is hereby incorporated byreference. Some optical design programmes such as Oslo® and Code V®allow optical systems to be described and designed through mathematicalfunctions, by which it is also possible to setnon-rotationally-symmetrical optical surfaces. The aforementionedmathematical descriptions relate to mathematical surfaces. An actuallyoptically used optical surface, i.e. the physical surface of an opticalelement, which surface is acted upon by an illumination beam and can bedescribed with this type of mathematical description, generally containsonly a portion of the actual mathematical surface, also known as theparent surface. The mathematical surface thus extends beyond thephysical optically effective surface. In so far as an optical system canbe described with the aid of a reference axis, some or all of theoptically used surface portions may be arranged beyond this referenceaxis in such a way that the reference axis divides the mathematicalsurface, but not, however, the actual optically used portion of thismathematical surface.

Field planes arranged parallel to one another facilitate the integrationof the imaging optical system into constructional surroundings. Thisadvantage can be particularly significant when the imaging opticalsystem is used in a scanning projection exposure installation, since thescan directions can then be guided parallel to one another.

A maximum angle of reflection of 25° (e.g., a maximum angle ofreflection of 20°, a maximum angle of reflection of 16°) can allow theimaging optical system to be used in a highly effective manner as aprojection optical system for an EUV projection exposure installation,since the mirrors, over the entire aperture, i.e. the entire usablereflective surface, thereof, may then be covered with consistentlyhighly reflective layers. This advantage can be important in particularfor the p-polarisation components of reflected radiation, since thereflectivity of p-polarisation components decreases rapidly in the caseof elevated angles of reflection.

An imaging optical system, wherein the quotient of a maximum angle ofreflection of the imaging light within the imaging optical system andthe numerical aperture thereof on the image side is at most 40°, canallow a good compromise to be achieved between high EUV throughput andoptimised pattern resolution in an EUV projection exposure installation.

A mirror arranged before the last mirror in the imaging light path inthe region of a pupil plane and having a convex basic shape allows goodPetzval correction of the imaging optical system to be achieved.

An imaging optical system having at least four mirrors (e.g., sixmirrors) can be particularly suitable for the construction of an imagingoptical system that is both compact and well-corrected in terms of itsimaging errors.

Imaging optical systems having mirrors with angular magnification of theprincipal ray, wherein at least two of the mirrors have a negativeangular magnification of the principal ray, and wherein a mirror withpositive angular magnification of the principle ray is arranged betweentwo mirrors with negative angular magnification of the principal ray,can allow systems with low maximum angles of reflection to be achieved.Imaging optical systems with three mirrors and a negative angularmagnification of the principal ray are also possible. The angularmagnification of the principal ray is defined as the angle between aprincipal ray belonging to a central field point and a reference axis.The reference axis is perpendicular to the object plane of theprojection exposure installation and extends through the centre point ofthe object field.

A beam angle of a central imaging beam, directed through the last mirrorand essentially through a pupil, of a central object point of greaterthan 85° relative to the image plane produces merely a low lateral imageshift in the image plane when defocusing.

An imaging optical system, wherein the imaging light path directedthrough the last mirror has an intermediate image being arranged in anintermediate image plane in the region of the through-hole in themirror, a portion of the optical system between the object plane and theintermediate image plane having a reducing magnification level of atleast 2× can allow a relatively large penultimate mirror in the lightpath before the image field to be used. This can reduce the maximumangle of reflection and can also reduce the extent of pupil obscurationif the penultimate mirror is obscured. It is also possible to achievemagnification of the portion of the optical system of greater than 2×(e.g., greater than 2.5×, greater than 3.0×, 3.2×).

An arrangement, wherein a mirror, which is arranged so as to be thepenultimate mirror in the imaging light path, has a through-hole forimaging light to pass through, the image plane being arranged behind thepenultimate mirror so as to be off-centre by not more than a fifth ofthe diameter of the penultimate mirror (e.g., to be central) relative tothe penultimate mirror, can allow a penultimate mirror with a relativelysmall through-hole to be used. This can ensure a stable penultimatemirror and low pupil obscuration.

A slightly curved penultimate mirror having a radius of curvaturegreater than 500 mm (e.g., greater than 1,000 mm, greater than 1,500 mm)can allow a small through-hole relative to the diameter of the mirror tobe achieved in the penultimate mirror at a given image-side numericalaperture.

An image field greater than 1 mm² can lead to good throughput when theimaging optical system is used in a projection exposure installation.

An image-side numerical aperture on the image side of at least 0.4(e.g., at least 0.45, at least 0.5, at least 0.55, at least 0.6, atleast 0.65, at least 0.7) can allow high resolution of the imagingoptical system to be achieved.

An image-side telecentric imaging optical system can allow, for example,the system to refocus in the image plane without thereby changing theimaging magnification and thus can increase the flexibility of use ofthe imaging optical system. On the object-side, the imaging opticalsystem can be formed in such a way that individual rays which areassociated with different object field points but with the same exposuredirection, enter the imaging optical system from the object field in aconvergent manner. Alternatively, it is also possible for the individualrays of this type to enter the imaging optical system in a divergent orparallel manner. The latter case results in an object-side telecentricimaging optical system.

A low object-image shift of less than 100 mm (e.g., less than 10 mm,less than 1 mm) can lead to a compact imaging optical system and, inaddition, facilitates optical system test methods, in which the imagingoptical system is rotated about an axis extending through the object orimage field and located perpendicular to the corresponding field plane,since the object or image field then does not shift too far duringrotation.

At least one pair of adjacent mirrors, wherein the mirrors are at adistance from one another, perpendicular to the object plane and/or tothe image plane, of more than 40% of the distance between the objectfield and the image field, can allow small angles of incidence to beobserved in the light path of the imaging light through the imagingoptical system. Due to the small angles of incidence, it is alsopossible to achieve highly reflective mirrors in the EUV wavelengthrange. In particular, 2, 3, 4 or more pairs of mirrors may satisfy thedistance condition.

Having in the imaging optical system at least one mirror with a minimumdistance of less than 25 mm from the reflection surface used to theclosest imaging light path not acting upon the mirror results in animaging optical system in which the angle of incidence on the mirrors iskept as small as possible. The advantages of small angles of incidenceon mirrors has previously been discussed. In particular 2, 3 or 4mirrors of the imaging optical system may be at the minimum distance.This minimum distance can be less than 25 mm, but optionally greaterthan 5 mm so the constructional demands on the mirrors are not toogreat.

An imaging optical system, wherein the imaging light is reflected to theimage field by the mirror including the through-hole for the imaginglight to pass through, in which the last mirror in the imaging lightpath includes the through-hole, can allow a high numerical aperture tobe achieved in a compact construction with minimised imaging errors.

The advantages of a projection exposure installation including animaging optical system, including a light source for the illuminationand imaging light, and including a lens system for directing theillumination light to the object field of the imaging optical system,and wherein the light source for generating the illumination light isformed with a wavelength of between 10 and 30 mm, can correspond tothose previously discussed with regard to the imaging optical system.The light source of the projection exposure installation may be in theform of a broadband light source and may have, for example, a bandwidthgreater than 1 nm (e.g., greater than 10 nm, greater than 100 nm). Inaddition, the projection exposure installation may be constructed insuch a way that it can be operated with light sources of differentwavelengths.

Corresponding advantages can also apply to the production methodincluding the steps of providing a reticle and a wafer, projecting astructure on the reticle onto a light-sensitive layer of the wafer byusing the projection exposure installation and producing amicrostructure on the wafer, and the microstructured component producedthereby.

Using the imaging optical system as a microlens, wherein the arrangementof the optical components when used in this way correspond to those onthe condition that object plane and image plane are exchanged, and wheninspecting a substrate which is to be exposed or has already beenexposed with respect to projection exposure with a lithographicprojection exposure installation, can result in the advantage that, inthe region of the intermediate image, drilling through any very smallmirrors can be avoided.

Embodiments of the disclosure will be described in the following ingreater detail with reference to the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically shows a projection exposure installation formicrolithography;

FIG. 2 shows a cross-section through a projection optical system of theprojection exposure installation in FIG. 1 containing field pointsspaced from one another along an imaging light path;

FIG. 3 shows a plan view of an image field of the projection opticalsystem in FIG. 2 viewed from direction III in FIG. 2;

FIG. 4 shows a cross-section through a non-rotationally-symmetricalfree-form surface and through a rotationally symmetrical surface;

FIG. 5 shows a cross-section through a portion of a mirror of theprojection optical system in FIG. 2;

FIG. 6 schematically shows a light path onto a mirror in the projectionoptical system in FIG. 2 with positive angular magnification of theprincipal ray;

FIG. 7 schematically shows a light path onto a mirror in the projectionoptical system in FIG. 2 with a negative angular magnification of theprincipal ray;

FIG. 8 shows a similar view to FIG. 1 of a projection exposureinstallation for microlithography;

FIG. 9 shows an enlarged partial detail of a wafer exposed with theprojection exposure installation in FIG. 1 or 8, and a mirror adjacentthereto;

FIG. 10 shows a view similar to that of FIG. 2 of a projection opticalsystem;

FIG. 11 shows a view similar to that of FIG. 2 of a projection opticalsystem;

FIG. 12 shows a view similar to that of FIG. 2 of a projection opticalsystem;

FIG. 13 shows a view similar to that of FIG. 11 of a microscope lens forinspecting wafers;

FIGS. 14 and 15 show two further views similar to that of FIG. 2 of aprojection optical system; and

FIGS. 16 and 17 show views similar to that of FIG. 13 of a microscopelens for inspecting wafers.

DETAILED DESCRIPTION

Referring to FIG. 1, a projection exposure installation 1 formicrolithography has a light source 2 for illumination light. The lightsource 2 is an EUV light source which produces light in a wavelengthrange of between 10 nm and 30 nm. Other EUV wavelengths are alsopossible. In general, even any desired wavelengths, for example visiblewavelengths, are possible for the illumination light guided in theprojection exposure installation 1. A light path of the illuminationlight 3 is very schematically shown in FIG. 1.

A lens system 5 serves to guide the illumination light 3 to an objectfield in an object plane 4. The object field is imaged by a projectionoptical system 6 in an image field 7 (cf. FIG. 3) in an image plane 8with a predetermined reduction scale. The projection optical system 6reduces the size by a factor of 8. Other imaging magnification levelsare also possible, for example 4×, 5×, 6× or even imaging magnificationlevels greater than 8×. An imaging magnification level of 8× isparticularly suitable for illumination light with an EUV wavelength,since the object-side angle of incidence on a reflection mask canthereby remain small. An image-side aperture of the projection opticalsystem of NA=0.5 produces an illumination angle of less than 6° on theobject-side. An image magnification level of 8× does not require, inaddition, unnecessarily large masks to be used. In the projectionoptical system 6 according to FIG. 2, the image plane 8 is arrangedparallel to the object plane 4. A portion of a reflective mask 9, alsoknown as a reticle, coinciding with the object field is hereby imaged.Imaging is achieved on the surface of a substrate 10 in the form of awafer which is supported by a substrate holder 11. In FIG. 1, a lightbeam 12 of the illumination light 3 entering the projection opticalsystem 6 is shown schematically between the reticle 9 and the projectionoptical system, and a ray beam 13 of the illumination light 3 exitingfrom the projection optical system 6 is shown schematically between theprojection optical system 6 and the substrate 10. The image fieldsidenumerical aperture of the projection optical system 6 in accordance withFIG. 2 is 0.50. The projection optical system 6 is telecentric on theimage side.

In order to aid the description of the projection exposure installation1 an xyz Cartesian coordinate system is provided in the drawings andshows the respective locations of the components represented in thefigures. In FIG. 1 the x direction extends perpendicularly into thedrawing plane, the y direction extends to the right and the z directionextends downwards.

The projection exposure installation 1 is a scanner-type device. Boththe reticle 9 and the substrate 10 are scanned in the y direction duringoperation of the projection exposure installation 1.

FIG. 2 shows the optical construction of the projection optical system6. The light path of each of three individual rays 14, coming from fiveobject field points which, in FIG. 2, are on top of one another and areat a distance from one another in the y direction, is shown, the threeindividual rays 14 which belong to one of the five object field pointseach being associated to three different illumination directions for thefive object field points.

From object field 4, the individual rays 14 are initially reflected by afirst mirror 15, which is denoted in the following as mirror M1, and aresubsequently reflected by further mirrors 16, 17, 18, 19, 20, which arealso denoted in the following as mirrors M2, M3, M4, M5 and M6 in thesequence of the light path. The projection optical system 6 in FIG. 2therefore has 6 reflective mirrors. The mirrors have a coating which ishighly reflective for the wavelength of the illumination light, ifrequired due to the wavelength, for example with EUV wavelengths.Radiation of greatly differing wavelengths may also be guided in thelens system 5 and the projection optical system 6, since these opticalsystems have substantially achromatic properties. In these opticalsystems it is therefore possible, for example, to direct an adjustinglaser or to operate an autofocusing system, at the same time using awavelength for the illumination light that differs greatly from theoperating wavelengths of the adjusting laser or the autofocusing system.An adjusting laser can thus operate at 632.8 nm, 248 nm or 193 nm,while, at the same time, an illumination light is operated in the rangebetween 10 and 30 nm.

The mirrors 15, 17 and 19 have a convex basic shape and can thus bedescribed by a convex best-fitted surface. The third mirror 17 inparticular has a convex basic shape. The mirrors 16, 18 and 20 have aconcave basic shape and can thus be described by a concave best-fittedsurface. In the following description, this type of mirror is referredto in a simplified manner merely as convex or concave. The convex mirror17 provides good Petzval correction in the projection optical system 6.

The individual rays 14, which come from spaced object field points andare associated with the same illumination direction, enter theprojection optical system 6 in a convergent manner between the objectplane 4 and the first mirror M1. The design of the projection opticalsystem 6 can be adapted in such a way that the same illuminationdirections for the individual rays 14 associated with the object fieldpoints also extend in a divergent manner from, or in a parallel mannerto, one another between these components. The latter variant correspondsto a telecentric light path on the object side.

The individual rays 14 belonging to a particular illumination directionof the five object field points 3 merge in a pupil plane 21 of theprojection optical system 6, adjacent to which the mirror 17 isarranged. The mirror 17 is therefore also known as a pupil mirror. Anaperture stop may be arranged in the pupil plane 21 for limiting theillumination light ray beam. The aperture stop may be provided by amechanical and removable stop or in the form of an appropriate coatingapplied directly to the mirror M3.

The mirrors 15 to 18 image the object plane 4 in an intermediate imageplane 22. The intermediate image-side numerical aperture of theprojection optical system 6 is 0.2. The mirrors 15 to 18 form a firstportion of the imaging optical system of the projection optical system 6with a reducing magnification level of 3.2×. The following mirrors 19and 20 form a further portion of the imaging optical system of theprojection optical system 6 with a reducing magnification level of 2.5×.In the sixth mirror 20, in the region of the intermediate image plane22, a through-hole 23 is formed, through which the illumination light 3passes after reflection by the fourth mirror 18 towards the fifth mirror19. In turn, the fifth mirror 19 has a central through-hole 24 throughwhich the ray beam 13 passes between the sixth mirror 20 and the imagefield 8.

The fifth mirror 19, which, together with the sixth mirror 20, imagesthe illumination light 3 from the intermediate image plane 22 in theimage plane 8, is arranged in the vicinity of a further pupil plane 25,conjugate to the first pupil plane 21, of the projection optical system6. The further pupil plane 25 is typically located in the light path ofthe illumination light 3 between the fifth mirror 19 and the sixthmirror 20, so there is a physically accessible stop plane at thelocation of the further pupil plane 25. An aperture stop canalternatively or additionally be arranged in this diaphragm plane, aspreviously described with respect to the aperture stop in the region ofthe pupil plane 21.

The projection optical system 6 has an obscuration stop arrangedcentrally in one of the pupil planes 21, 25. By this means the beamportions of the projection light path, associated with the centralthrough-holes 23, 24 in the mirrors 20, 19, are obscured. Theconstruction of the projection optical system 6 can therefore also betermed construction with central pupil obscuration.

A marked individual ray 14, which connects a central object field pointto a centrally illuminated point in the entrance pupil of the projectionoptical system 6 in the entrance pupil plane 21, will also be referredto in the following as the principal ray 26 of a central field point.The principal ray 26 of the central field point makes approximately aright angle with the image plane 8 after reflection on the sixth mirror20 and thus extends approximately parallel to the z-axis of theprojection exposure installation 1. The angle is greater than 85° in anycase.

The image field 7 is rectangular. The aspect ratio of the image field 7is not shown to scale in FIG. 3. The image field 7 extends by 13 mmparallel to the x direction. The image field 7 extends by 1 mm parallelto the y direction. The image field 7 is located centrally behind thefifth mirror 19, as shown in FIG. 3. A radius R of the through-hole 24can be calculated from:

$R = {{\frac{1}{2} \cdot D} + {d_{w} \cdot {{NA}.}}}$

D is the diagonal of the image field 7. d_(w) is the working distance ofthe mirror 19 from the image plane. NA is the numerical aperture on theimage side.

All six mirrors 15 to 20 of the projection optical system 6 are in theform of free-form surfaces which cannot be described by a rotationallysymmetrical function. Other configurations of the projection opticalsystem 6 are also possible, in which at least one of the mirrors 15 to20 includes a free-form reflection surface of this type.

Production of a free-form surface 27 of this type from a rotationallysymmetrical reference surface 28 will be described in the following withreference to FIG. 4. First of all, information on the characterisationof the free-form surface under consideration is obtained. The referencesurface 28 can, for example, be a rotationally symmetrical asphere. Partof the design information may be the radius of curvature of thereference surface 28, which is also referred to as 1/c, c denoting thevertex curvature of the reference surface 28. A conical constant k ofthe reference surface 28 and polynomial coefficients which describe thereference surface 28 are also part of the information.

Alternatively or additionally, the information characterising thereference surface 28 can also be obtained from a surface measurement ofa reference mirror surface, for example, by using an interferometer.This type of surface measurement produces a function z′(x′, y′), whichdescribes the reference surface 28, z′ denoting the rising height of thereference surface 28 along the z′-axis for different (x′, y′)coordinates, as shown in FIG. 4.

This first step in designing the free-form surface also includesdetermining the portion of the mirror surface, which is only defined bythe surface description and is initially unlimited, that will actuallybe used for reflecting illumination light 3 during imaging of the objectfield in the image field 7. The region is also referred to as thefootprint.

The footprint of the mirror can be at least approximately determined byray tracing of the projection optical system 6. Examples for a possiblefootprint in the x dimension are provided in FIG. 4. x_(min) refers tothe lower limit and x_(max) refers to the upper limit for the exemplaryfootprint. The data above x_(max) and below x_(min) are similarlycalculated within specific limits so that no undesired edge effectsarise when determining the free-form surface 27.

After the information characterising the reference surface 28 has beendetermined, a local coordinate system for the reference surface 28 isintroduced, in which both decentration and tilting of the referencesurface 28 are zero. The z′-axis is thus the axis of rotational symmetryof the aspherical reference surface 28 or, if the reference surface wasobtained by a surface measurement, the optical axis of the measuringdevice, for example the interferometer. The z′-axis is generallydisplaced parallel to and tilted relative to the z-axis of the xyzcoordinate system of the projection exposure installation 1. This alsoapplies to the other coordinate axes x′, y′. This parallel displacementor tilting is determined in the initial step in the optical design ofthe free-form surface.

As an alternative to an asphere, the reference surface 28 may also be aspherical surface. The origin of the coordinates x_(c), y_(c), z_(c) fordescribing the spherical reference surface 28 generally differs from theorigin of the xyz coordinate system of the projection exposureinstallation 1.

After the reference surface 28 has been determined, a local distanced_(i) (i=1 . . . N) between a number of points on the reference surface28 and points on the free-form surface 27 parallel to the z′-axis isdetermined. The different local distances d_(i) are then varied until aset of secondary conditions is satisfied. The secondary conditions arepredetermined limit values for specific imaging errors and/orillumination properties of the projection optical system 6.

The free form surface can be mathematically described by the followingequation:

in which:

$Z = {\frac{{cr}^{2}}{1 + \sqrt{1 - {\left( {1 + k} \right)c^{2}r^{2}}}} + {\sum\limits_{j = 2}^{66}\;{C_{j}X^{m}Y^{n}}}}$$j = {\frac{\left( {m + n} \right)^{2} + m + {3n}}{2} + 1}$

Z is the rising height of the free-form surface parallel to a Z-axiswhich can, for example be parallel to the z′-axis in FIG. 4.

c is a constant corresponding to the vertex curvature of a correspondingasphere. k corresponds to a conical constant of a corresponding asphere.C_(j) are the coefficients of the monomials X^(m)Y^(n). The values of c,k and C_(j) are generally determined on the basis of the desired opticalproperties of the mirror inside the projection optical system 6. Theorder of the monomial, m+n, can be varied as desired. A monomial of ahigher order can lead to a design of the projection optical system withimproved image error correction, but is, however, more complex tocalculate. m+n can have values of between 3 and more than 20.

Free-form surfaces can also be described mathematically by Zernikepolynomials, which are described, for example, in the manual of theoptical design program CODE V®. Alternatively, free-form surfaces can bedescribed with two-dimensional spline surfaces. Examples thereof areBezier curves or non-uniform rational basis splines (NURBS).Two-dimensional spline surfaces can be described, for example, by a gridof points in an xy-plane and related z-values, or by the points andtheir related gradients. Depending on the respective type of splinesurface, the complete surface will be obtained by interpolating betweenthe grid points by using, for example, polynomials or functions whichhave specific properties with respect to their continuity anddifferentiability. Examples thereof include analytical functions.

The mirrors 15 to 20 have multiple reflective coatings for optimisingthe reflection thereof for the incident EUV illumination light 3.Reflection is better the closer the angle of incidence of the individualrays 14 on the mirror surface is to the perpendicular incidence. Theprojection optical system 6 has very small angles of reflection for allof the individual rays 14. Half of the angle between the individual ray14 striking a point on one of the mirrors 15 to 20 and the individualray 14 reflected from this point will be referred to in the following asthe angle of reflection of this point.

The maximum angle of reflection in the projection optical system 6 isthe angle of the individual ray 14 at the outer edge of the fifth mirror19. This angle α is approximately 16° in the projection optical system6. The quotient of the maximum angle of reflection α and the numericalaperture is thus 32° in the projection optical system 6 shown in FIG. 2.

The dependence of the size of the angle of reflection on the position ofthe point of incidence on the reflection mirror will be explainedschematically in the following with an example of a sample reflectionmirror 29, shown in FIG. 5. In the picture a divergent beam ofindividual rays 14 a, 14 b, 14 c strikes a reflection surface 30 of thesample reflection mirror 29. The reflection surface 30 is convex. Due tothe collective effect of the reflection surface 30, the incidentdescending beam made of individual rays 14 a, 14 b and 14 c is deflectedforming a reflected convergent beam. The individual ray 14 a strikingclosest to the edge on the reflection surface 30 is deflected with thelargest angle of reflection α, the centre individual ray 14 b isdeflected with an angle of reflection β which is smaller in comparisonthereto and the individual ray 14 c furthest from the edge of the samplereflection mirror 29 is deflected by the smallest angle of reflection γ.

The light path within the projection optical system 6 can additionallybe characterised by the sequence of angular magnification of theprincipal ray. This will be explained in the following with reference tothe schematic drawings 6 and 7. In FIG. 6, the principal ray 26 isradiated onto a sample reflection mirror 31 at an angle α to a referenceaxis 32 extending perpendicular to the object plane 4 of the projectionexposure installation 1. On the side of the object field, i.e. up to andinclusive of the mirror M4, the reference axis 32 is additionallydefined by the centre of the object field. The reference axis 32generally does not coincide with the z-axis but runs parallel to theaxis. After being reflected by the sample reflection mirror 31, theprincipal ray 26 is reflected back at an angle β to the reference axis32. Since both angles α, β are between 0 and 90°, the quotient tan α/tanβ is positive. The sample reflection mirror 31 therefore has a positiveangular magnification of the principal ray.

FIG. 7 shows the case of negative angular magnification of the principalray. The incident principal ray 26 intersects the reference axis 32 atan angle α which is between 0 and 90°. The principal ray 26 reflected bya sample reflection mirror 33 virtually encloses an angle β between 90and 180° with the reference axis 32. In this case the quotient tan α/tanβ is thus negative.

In the projection optical system 6, the first mirror 15 has negativeangular magnification of the principal ray. The second mirror 16 haspositive angular magnification of the principal ray. The third mirror 17has negative angular magnification of the principal ray. The angularmagnification of the fourth mirror 18 is infinite, since the angle β is180° at that location.

FIG. 8 again shows a slightly modified representation of the projectionexposure installation 1 for clearly showing a further characterisingvalue of the projection optical system 6, namely the object-image shiftd_(ois). This is defined as the distance between a perpendicularprojection of the central object point onto the image plane 8 and thecentral image point. In the projection optical system 6 shown in FIG. 2,the object-image shift d_(ois) is less than 1 mm.

FIG. 9 demonstrates a further characteristic of the projection opticalsystem 6, namely the free working distance d_(w). This is defined as thedistance between the image plane 8 and the closest portion 34 thereto ofone of the mirrors of the projection optical system 6, i.e. mirror 19 inthe embodiment shown in FIG. 2.

In the projection optical system 6, the free working distance d_(w) is40 mm. The fifth mirror 19, which is closest to the image plane 8, cantherefore be constructed having a thickness that provides sufficientstability of the fifth mirror 19. Materials for mirrors of this typeinclude, for example, quartz, zerodur or silicon carbide compounds.Other materials with ultra low expansion properties may also be used.Examples of materials of this type are known from products sold byCorning, USA, under the name “ULE”.

The optical data of the projection optical system 6 are summarised inthe following:

The image-side numerical aperture NA is 0.5. The size of the image fieldis 1×13 mm². The reducing magnification level is 8×. The image field 7is rectangular. The wavelength of the illumination light is 13.5 nm. Thesequence of the optical effects of the mirrors M1 to M6 (negative N;positive P) is NPNPNP. Principal rays enter the projection opticalsystem in a convergent manner from the object plane. An aperture stop isarranged on the mirror M3 for limiting the illumination light at theedge. The z-distance between the object plane 4 and the image plane 8 is1,500 mm. The object-image shift is 0.42 mm. 5.9% of the illuminatedsurfaces in the pupil planes are obscured. The projection optical systemhas a wave front error (rms) of 0.02 in units of the wavelength of theillumination light 3. The distortion is 12 nm. The field curvature is 9nm. The angle of the principal ray at the central object field point is5.9°. The mirror M1 has dimensions (x/y) of 117×61 mm². The mirror M2has dimensions of 306×143 mm². The mirror M3 has dimensions of 80×77mm². The mirror M4 has dimensions of 174×126 mm². The mirror M5 hasdimensions of 253×245 mm². The mirror M6 has dimensions of 676×666 mm².The sequence of the principal ray angle of incidence, of the principalray 26 of the central object field point, on the mirrors M1 to M6 is16.01°, 7.14° 13.13°, 7.21°, 0.0° and 0.0°. The sequence of the maximumangle of incidence on the mirrors M1 to M6 is 22.55°, 9.62°, 13.90°,10.16°, 16.23°, 4.37°. The sequence of bandwidths of the angle ofincidence on the mirrors M1 to M6 is 13.12°, 5.07°, 1.58°, 6.10°, lessthan 16.23° and less than 4.37°. The working distance in the objectplane 4 is 100 mm. The working distance in the image plane 8 is 40 mm.The ratio of the distance between the object plane 4 and the mirror M1and the distance between the object plane 4 and the mirror M2 is 4.25.Between each of the adjacent mirrors M2-M3, M4-M5, M5-M6 and alsobetween the mirror M6 and the image plane 8 there is a distance ofgreater than 40% of the z-distance between the object plane 4 and theimage plane 8. The mirrors M1 and M4 have a minimum distance from theused reflection surface to the closest imaging light path not acting onthe mirror (free board) of less than 25 mm.

The optical design data of the reflection surfaces of the mirrors M1 toM6 of the projection optical system 6 can be gathered from the followingtables. The first of the tables shows the respective reciprocal value ofthe vertex curvature (radius) and a distance value (thickness), whichcorresponds to the z-distance of adjacent elements in the light path,starting from the object plane, for the optical components and theaperture stop. The second table shows the coefficients C_(j) of themonomials X^(m)Y^(n) in the aforementioned free-form surface equationfor the mirrors M1 to M6. At the end of the second table the value bywhich the respective mirror is decentred (Y-decenter) and rotated(X-rotation) from a mirror reference design is given in millimetres.This corresponds to the parallel displacement and tilting in thefree-form surface design method described above. Displacement thus takesplace in the y direction and tilting takes place about the x axis. Theangle of rotation is given in degrees.

Surface Radius Thickness Mode Object INFINITY 425.420 Mirror 1 294.947−325.420 REFL Mirror 2 681.039 690.757 REFL Mirror 3 319.431 0.000 REFLSTOP INFINITY −244.337 Mirror 4 396.876 913.580 REFL Mirror 5 1749.322−620.710 REFL Mirror 6 834.214 660.710 REFL Image INFINITY 0.000

Coefficient M1 M2 M3 M4 M5 M6 K −8.972907E−01   −2.722153E−01  6.009025E+00 −2.083103E−01   3.438760E+01 3.027724E−01 Y 0.000000E+000.000000E+00 0.000000E+00 0.000000E+00 0.000000E+00 0.000000E+00 X2−6.723739E−04   −9.397624E−05   −5.616960E−04   −1.596929E−04  9.008585E−06 3.436254E−05 Y2 −7.259542E−04   −1.245430E−04  −4.962660E−04   −1.209634E−04   2.711820E−06 3.328586E−05 X2Y−2.611787E−07   9.438147E−09 −6.471824E−07   −1.397345E−07  −3.166886E−08   4.403654E−10 Y3 1.848873E−07 2.540415E−08 4.939085E−07−1.842875E−07   5.311486E−09 6.726500E−10 X4 −5.585253E−10  −3.707750E−11   −2.414232E−08   −1.057114E−10   9.436063E−101.898115E−12 X2Y2 −1.454988E−09   −1.447610E−10   −4.434814E−08  −5.420267E−11   1.946694E−09 4.974829E−12 Y4 −5.523329E−09  −2.392090E−11   −1.815299E−08   4.380159E−10 9.997897E−10 3.488151E−12X4Y −1.364069E−12   1.084325E−15 −3.114225E−11   −1.197000E−12  5.182403E−14 4.428526E−16 X2Y3 6.732062E−12 1.382697E−13 9.802932E−11−1.950774E−12   4.779360E−13 1.769320E−16 Y5 3.635430E−11 0.000000E+007.767198E−11 −1.559300E−12   3.401358E−13 4.373202E−16 X6−2.750390E−15   −9.087823E−17   −9.415776E−13   −4.463189E−16  1.620585E−15 9.932173E−19 X4Y2 2.324635E−14 −5.352295E−17  −3.094331E−12   7.684993E−15 5.526453E−15 3.332327E−18 X2Y4 3.956161E−15−2.030722E−16   −3.217471E−12   3.107748E−15 5.847027E−15 3.759258E−18Y6 −1.092384E−13   −8.567898E−17   −7.281446E−13   −7.204126E−16  1.552120E−15 1.038153E−18 X6Y −1.179589E−16   4.377060E−19−1.789065E−16   −1.451963E−19   3.245847E−19 1.324484E−22 X4Y3−2.570887E−16   0.000000E+00 1.023466E−14 −4.269245E−17   1.564405E−18−9.051915E−22   X2Y5 −8.917936E−17   7.695621E−21 1.492914E−14−1.217398E−17   2.326082E−18 −1.811267E−22   Y7 1.236168E−160.000000E+00 4.771084E−15 5.163018E−18 7.533041E−19 2.904675E−22 X87.305784E−20 −2.087892E−22   −4.992347E−17   −9.852110E−22  5.510114E−21 −9.878544E−26   X6Y2 6.107242E−19 −8.775175E−22  −2.298846E−16   −2.713369E−20   2.453885E−20 6.254655E−25 X4Y45.443174E−19 −2.629666E−22   −3.296922E−16   3.809184E−20 3.817638E−204.270350E−24 X2Y6 −6.091249E−19   −8.692919E−23   −1.689920E−16  3.730606E−21 2.483560E−20 4.657493E−24 Y8 −2.536724E−19   8.059798E−24−2.318537E−17   −7.839829E−21   6.692413E−21 1.504196E−24 X8Y0.000000E+00 0.000000E+00 0.000000E+00 0.000000E+00 0.000000E+000.000000E+00 X6Y3 0.000000E+00 0.000000E+00 0.000000E+00 0.000000E+000.000000E+00 0.000000E+00 X4Y5 0.000000E+00 0.000000E+00 0.000000E+000.000000E+00 0.000000E+00 0.000000E+00 X2Y7 0.000000E+00 0.000000E+000.000000E+00 0.000000E+00 0.000000E+00 0.000000E+00 Y9 0.000000E+000.000000E+00 0.000000E+00 0.000000E+00 0.000000E+00 0.000000E+00 X100.000000E+00 0.000000E+00 0.000000E+00 0.000000E+00 0.000000E+000.000000E+00 X8Y2 0.000000E+00 0.000000E+00 0.000000E+00 0.000000E+000.000000E+00 0.000000E+00 X6Y4 0.000000E+00 0.000000E+00 0.000000E+000.000000E+00 0.000000E+00 0.000000E+00 X4Y6 0.000000E+00 0.000000E+000.000000E+00 0.000000E+00 0.000000E+00 0.000000E+00 X2Y8 0.000000E+000.000000E+00 0.000000E+00 0.000000E+00 0.000000E+00 0.000000E+00 Y100.000000E+00 0.000000E+00 0.000000E+00 0.000000E+00 0.000000E+000.000000E+00 Nradius   1.00E+00   1.00E+00   1.00E+00   1.00E+00  1.00E+00   1.00E+00 Y-decenter 68.139 −264.855 176.907 −28.983 101.91497.590 X-rotation −1.258 25.000 1.304 25.000 0.098 0.498

FIG. 10 shows a projection optical system 35, which can be used, insteadof the projection optical system 6, in the projection exposureinstallation 1. Components or reference quantities which correspond tothose which have previously been described with reference to FIGS. 1 to9 have the same reference numerals and will not be discussed in detailagain.

The projection optical system 35 also has a total of six reflectivemirrors, which, starting from the object plane 4 in the light pathsequence, have reference numerals 36 to 41, and will also be referred toin the following as mirrors M1 to M6. The mirrors 36 to 41 all havereflective free-form surfaces which cannot be described by arotationally symmetrical function. The mirrors 36, 38 and 40 have aconvex basic shape and the mirrors 37, 39, 41 have a concave basicshape.

The projection optical system 35 has a reduction factor of 8. Theprojection optical system 36 has an image-side numerical aperture of0.5. The dimensions of the image field 7 of the projection opticalsystem 35 are exactly the same as those of the projection optical system6. The intermediate image-side numerical aperture is 0.28.

The first mirror 36 has negative angular magnification of the principalray. The second mirror 37 has positive angular magnification of theprincipal ray. The third mirror 38 has negative angular magnification ofthe principal ray. The fourth mirror 39 has infinite angularmagnification of the principal ray since the principal ray 26 extendsfrom the fourth mirror 39 so as to be perpendicular to the image plane8.

In the projection optical system 35, the object-image shift is markedlygreater than in the projection optical system 6 and is 134 mm.

The maximum angle of reflection α, which is also achieved by the rays atthe edge of the fifth mirror 40 in the projection optical system 35, is17°. The quotient of the maximum angle of reflection α and theimage-side numerical aperture is 34°.

At 42 mm, the free working distance d_(w) in the projection opticalsystem 35 is comparable with the free working distance of the projectionoptical system 6.

The optical data of the projection optical system 35 are summarisedagain in the following:

The image-side numerical aperture NA is 0.5. The dimensions of the imagefield 7 are 1×13 mm². The reducing magnification level is 8×. The imagefield 7 is rectangular. The wavelength of the illumination light 3 is13.5 nm. The sequence of the optical effects of the mirrors M1 to M6(negative N; positive P) is PPNPNP. At the image-side, the projectionoptical system 35 is virtually telecentric. An aperture stop forlimiting the illumination light at the edge is arranged on mirror M3.The z-distance between the object plane 4 and the image plane 8 is 1,823mm. The object-image shift is 134 mm. 9.2% of the surfaces illuminatedin the pupil planes are obscured. The angle of the principal ray at thecentral object field point is 6°. The mirror M1 has dimensions (x/y) of241×138 mm². The mirror M2 has dimensions of 377×269 mm². The mirror M3has dimensions of 80×75 mm². The mirror M4 has dimensions of 246×197mm². The mirror M5 has dimensions of 352×304 mm². The mirror M6 hasdimensions of 776×678 mm². The sequence of the angle of incidence of theprincipal ray of the central object field point on the mirrors M1 to M6is 7.10°, 5.19°, 13.66°, 4.60°, 0.0° and 0.02°. The sequence of themaximum angle of incidence on the mirrors M1 to M6 is 12.23°, 5.53°,15.43°, 7.33°, 16.98° and 5.51°. The sequence of the bandwidths of theangle of incidence on the mirrors M1 to M6 is 9.93°, 0.78°, 2.98°,5.27°, less than 16.98° and less than 5.51°. The working distance in theobject plane 4 is 336 mm. The working distance in the image plane 8 is42 mm. The ratio of the distance between the object plane 4 and themirror M1 and the distance between the object plane 4 and the mirror M2is 3.04. The mirrors M1 to M4 have a minimum distance between the usedreflection surface and the closest imaging light path which does not actupon the mirror (free board) of less than 25 mm. The distance betweenthe object plane 4 and the mirror M1 and the distances between the pairsof mirrors M2-M3 and M4-M5 is greater than 40% of the distance betweenthe object plane and the image plane.

The optical design data of the reflection surfaces of the mirrors M1 toM6 of the projection optical system 35 can be gathered from thefollowing tables, which correspond to the tables for the projectionoptical system in accordance with FIG. 2.

Surface Radius Thickness Mode Object INFINITY 1023.157 Mirror 1−50610.892 −686.714 REFL Mirror 2 1171.238 828.471 REFL Mirror 3 318.0040.000 REFL STOP INFINITY −378.086 Mirror 4 413.560 994.620 REFL Mirror 52997.146 −612.464 REFL Mirror 6 817.300 654.356 REFL Image INFINITY0.000

M1 M2 M3 M4 M5 M6 Coeff. K 4.156869E+03 4.620221E−02 9.990462E+00−9.081861E−03   −2.372322E−01   6.789706E−03 Y 0.000000E+00 0.000000E+000.000000E+00 0.000000E+00 0.000000E+00 0.000000E+00 X2 4.219939E−05−7.203705E−05   −2.856541E−04   −1.831594E−04   −4.114605E−05  2.674563E−06 Y2 −2.952066E−04   −8.835077E−05   1.576757E−04−1.812758E−04   −3.733421E−05   7.346415E−06 X2Y −2.987815E−08  1.958263E−08 4.843132E−07 −7.966262E−08   −5.183892E−08  −3.629397E−09   Y3 5.768104E−07 8.430075E−08 −7.326854E−08  −9.457440E−08   −2.814518E−08   9.209304E−11 X4 2.110770E−102.081353E−11 1.569949E−08 −3.236129E−10   3.542926E−11 1.915378E−12 X2Y23.100857E−10 −1.622544E−11   3.080477E−08 −6.357050E−10   8.409285E−114.860251E−12 Y4 −2.322578E−10   −4.348550E−11   −9.859142E−09  −1.882466E−10   −2.084652E−11   6.490959E−14 X4Y 0.000000E+00−7.908907E−15   0.000000E+00 1.810068E−13 1.675236E−13 −2.002515E−15  X2Y3 0.000000E+00 1.426458E−14 0.000000E+00 −2.244745E−13   1.805451E−13−1.799322E−15   Y5 0.000000E+00 −1.321548E−14   0.000000E+00−2.730307E−13   −1.337121E−14   3.920622E−16 X6 0.000000E+000.000000E+00 0.000000E+00 0.000000E+00 0.000000E+00 0.000000E+00 X4Y20.000000E+00 0.000000E+00 0.000000E+00 0.000000E+00 0.000000E+000.000000E+00 X2Y4 0.000000E+00 0.000000E+00 0.000000E+00 0.000000E+000.000000E+00 0.000000E+00 Y6 0.000000E+00 0.000000E+00 0.000000E+000.000000E+00 0.000000E+00 0.000000E+00 X6Y 0.000000E+00 0.000000E+000.000000E+00 0.000000E+00 0.000000E+00 0.000000E+00 X4Y3 0.000000E+000.000000E+00 0.000000E+00 0.000000E+00 0.000000E+00 0.000000E+00 X2Y50.000000E+00 0.000000E+00 0.000000E+00 0.000000E+00 0.000000E+000.000000E+00 Y7 0.000000E+00 0.000000E+00 0.000000E+00 0.000000E+000.000000E+00 0.000000E+00 X8 0.000000E+00 0.000000E+00 0.000000E+000.000000E+00 0.000000E+00 0.000000E+00 X6Y2 0.000000E+00 0.000000E+000.000000E+00 0.000000E+00 0.000000E+00 0.000000E+00 X4Y4 0.000000E+000.000000E+00 0.000000E+00 0.000000E+00 0.000000E+00 0.000000E+00 X2Y60.000000E+00 0.000000E+00 0.000000E+00 0.000000E+00 0.000000E+000.000000E+00 Y8 0.000000E+00 0.000000E+00 0.000000E+00 0.000000E+000.000000E+00 0.000000E+00 X8Y 0.000000E+00 0.000000E+00 0.000000E+000.000000E+00 0.000000E+00 0.000000E+00 X6Y3 0.000000E+00 0.000000E+000.000000E+00 0.000000E+00 0.000000E+00 0.000000E+00 X4Y5 0.000000E+000.000000E+00 0.000000E+00 0.000000E+00 0.000000E+00 0.000000E+00 X2Y70.000000E+00 0.000000E+00 0.000000E+00 0.000000E+00 0.000000E+000.000000E+00 Y9 0.000000E+00 0.000000E+00 0.000000E+00 0.000000E+000.000000E+00 0.000000E+00 X10 0.000000E+00 0.000000E+00 0.000000E+000.000000E+00 0.000000E+00 0.000000E+00 X8Y2 0.000000E+00 0.000000E+000.000000E+00 0.000000E+00 0.000000E+00 0.000000E+00 X6Y4 0.000000E+000.000000E+00 0.000000E+00 0.000000E+00 0.000000E+00 0.000000E+00 X4Y60.000000E+00 0.000000E+00 0.000000E+00 0.000000E+00 0.000000E+000.000000E+00 X2Y8 0.000000E+00 0.000000E+00 0.000000E+00 0.000000E+000.000000E+00 0.000000E+00 Y10 0.000000E+00 0.000000E+00 0.000000E+000.000000E+00 0.000000E+00 0.000000E+00 Nradius   1.00E+00   1.00E+00  1.00E+00   1.00E+00   1.00E+00   1.00E+00 Coefficient Y-decenter−242.949 −116.015 15.485 −94.370 162.630 −15.854 X-rotation −2.051 4.274−4.892 10.143 −3.797 −2.652

FIG. 11 shows a further configuration of a projection optical system 42which may be used, instead of the projection optical system 6, in theprojection exposure installation 1. Components or reference quantitieswhich correspond to those which have previously been explained withreference to FIGS. 1 to 10 have the same reference numerals and will notagain be discussed in detail.

The projection optical system 42 also has six reflection mirrors whichare denoted by the reference numerals 43 to 48 in accordance with theirsequence in the imaging light path, starting from the object plane 4.The mirrors will also be referred to in the following as M1 to M6. Inthe projection optical system 42, all of the reflection surfaces areformed as free-form surfaces which cannot be described by a rotationallysymmetrical function.

The first mirror 43 is concave, but has only a very slight curve so thatit can be simply modified to form a mirror with a zero base curve or toform a convexly curved mirror. The second mirror 44 is concave and thethird mirror 45 is convex. The fourth mirror 46 is concave. The fifthmirror 47 is convex. The sixth mirror 48 is concave.

Each of the first three mirrors 43 to 45 has negative angularmagnification of the principal ray. The angular magnification of theprincipal ray of the fourth mirror 46 is infinite since the principalray 26 extends perpendicular to the image plane 8 after reflection bythe fourth mirror 46.

The projection optical system 42 has an image-side numerical aperture of0.5. The projection optical system 42 has an intermediate image-sidenumerical aperture of 0.11.

In the projection optical system 42, the free working distance d_(w) is20 mm.

The projection optical system 42 has a reduction factor of 8.

The dimensions of the image field in the projection optical system 42correspond to those of the projection optical systems 6 and 35.

In the projection optical system 42, the maximum angle of reflectionalso occurs in the outer edge rays reflected on the fifth mirror 47 andis α=16°. The quotient of the maximum angle of reflection of theillumination light 3 within the projection optical system 42 and theimage-side numerical aperture is 32.

The optical data of the projection optical system 42 are againsummarised in the following:

The image-side numerical aperture NA is 0.5. The dimensions of the imagefield are 1×13 mm². The reducing imaging magnification level is 8×. Theimage field 7 is rectangular. The wavelength of the illumination light 3is 13.5 nm. The sequence of the optical effects of the mirrors M1 to M6(negative N; positive P) is PPNPNP. Principal rays enter convergentlyinto the projection optical system 42 from the object plane 4. Anaperture stop is arranged on the mirror M2 for limiting the illuminationlight at the edge. The z-distance between the object plane 4 and theimage plane 8 is 1,700 mm. The object-image shift is 393 mm. 17.0% ofthe surfaces illuminated in the pupil planes are obscured. Theprojection optical system 42 has a wavefront error (rms) of 0.100 inunits of the wavelength of the illumination light 3. The distortion is16 nm. The image field curvature is 35 nm. The angle of the principalray at the central object field point is 6°. The mirror M1 hasdimensions (x/y) of 164×134 mm². The mirror M2 has dimensions of 312×170mm². The mirror M3 has dimensions of 147×155 mm². The mirror M4 hasdimensions of 354×196 mm². The mirror M5 has dimensions of 103×96 mm².The mirror M6 has dimensions of 457×444 mm². The sequence of theprincipal ray angle of incidence of the principal ray 26 of the centralobject field point on the mirrors M1 to M6 is 3.54°, 5.15°, 9.11°,4.45°, 0.01° and 0.01°. The sequence of the maximum angle of incidenceon the mirrors M1 to M6 is 6.18°, 5.62°, 9.80°, 6.85°, 15.94°, and2.36°. The sequence of the bandwidths of the angle of incidence on themirrors M1 to M6 is 5.16°, 1.08°, 1.52°, 4.63°, less than 15.94° andless than 2.38°. The working distance in the object plane 4 is 200 mm.The working distance in the image plane 8 is 20 mm. The ratio of thedistance between the object plane 4 and the mirror M1 and the distancebetween the object plane 4 and the mirror M2 is 5.07. The mirrors M1 andM2 have a minimum distance between the used reflection surface and theclosest imaging light path which does not act upon the mirror (freeboard) of less than 25 mm. The distance between the object plane 4 andthe mirror M1 and the distances between the pairs of mirrors M1-M2,M2-M3, M3-M4 and M4-M5 are greater than 40% of the distance between theobject plane and the image plane.

The optical design data for the reflection surfaces of the mirrors M1 toM6 of the projection optical system 42 can be gathered from thefollowing tables, which correspond to the tables previously provided forthe projection optical system 6 in accordance with FIG. 2.

Surface Radius Thickness Mode Object INFINITY 1014.317 Mirror 1−2704.152 −814.317 REFL Mirror 2 531.833 0.000 REFL STOP INFINITY935.139 Mirror 3 491.748 −718.533 REFL Mirror 4 870.221 1263.419 REFLMirror 5 245.485 −424.886 REFL Mirror 6 495.477 444.861 REFL ImageINFINITY 0.000

M1 M2 M3 M4 M5 M6 Coeff. K 1.144605E+01 −9.050341E−01   −1.089239E+00  −6.248739E−01   2.948620E+00 1.091603E−01 Y 0.000000E+00 0.000000E+000.000000E+00 0.000000E+00 0.000000E+00 0.000000E+00 X2 2.857150E−04−5.920234E−04   −2.610462E−04   −1.368396E−04   −4.475618E−04  1.730506E−05 Y2 9.176083E−05 −8.321210E−04   −7.892918E−04  −2.573840E−04   −4.405714E−04 1.563424E−05 X2Y 7.455682E−07−9.307510E−09   4.809832E−08 3.116002E−08 4.341012E−09 −3.269435E−09  Y3 4.605832E−08 −1.943924E−07   −2.212409E−07   7.169569E−09−4.274845E−07   −4.266065E−09   X4 −3.659110E−10   −1.644174E−11  8.510237E−10 1.713005E−11 2.190981E−09 1.081076E−11 X2Y2 −1.689952E−09  −3.435735E−10   6.957800E−12 −9.146320E−12   −5.946668E−09  2.690241E−11 Y4 −2.561746E−10   −6.556489E−10   1.590530E−103.880664E−13 −1.024229E−08   −1.427930E−12   X4Y −3.302144E−12  −1.451447E−13   −3.859663E−12   4.923124E−14 −2.729947E−11  −2.149830E−14   X2Y3 −2.296129E−12   2.463662E−13 4.902075E−124.230604E−14 −2.255029E−11   5.867643E−15 Y5 4.869118E−13 2.042378E−12−5.901335E−13   −2.503638E−15   −1.535539E−11   1.362505E−14 X62.532299E−14 3.607331E−16 −3.635906E−15   −1.910942E−17   5.572070E−141.020771E−17 X4Y2 1.050746E−14 3.556935E−15 6.819544E−14 1.635726E−164.514505E−13 3.413101E−17 X2Y4 2.215727E−14 8.029448E−15 −1.161921E−14  −1.548212E−17   −4.560072E−13   −1.111206E−17   Y6 −9.649794E−16  7.587037E−15 4.555774E−16 1.222675E−17 −3.875470E−13   −2.539409E−17  X6Y −1.936844E−16   −4.478100E−19   9.189317E−17 3.837055E−19−3.689123E−15   −6.718113E−20   X4Y3 5.354672E−17 1.140666E−17−4.339139E−16   2.254755E−19 −3.854918E−15   7.351666E−20   X2Y5−3.646598E−17   3.260549E−17 −2.644153E−17   −8.425001E−20  9.184510E−16 1.186287E−19 Y7 6.063079E−19 9.615056E−17 1.324974E−18−1.850786E−21   −2.798829E−15   6.587133E−20 X8 5.617315E−191.744698E−21 8.327575E−19 2.970358E−21 4.324289E−18 5.187555E−23 X6Y25.094397E−19 3.594344E−20 −9.344050E−19   2.069107E−21 4.500525E−173.412692E−23 X4Y4 −2.079112E−19   1.260510E−19 1.229358E−18−7.743007E−23   −2.240628E−17   6.720118E−23 X2Y6 −1.595633E−20  1.627768E−19 −2.763971E−20   1.708991E−22 −4.013864E−17   2.519384E−23Y8 1.940634E−21 4.827783E−19 5.031625E−21 −1.299209E−23  −6.317984E−18   −9.073694E−23   X8Y −3.793003E−21   2.116730E−23−7.801057E−21   1.432927E−23 −4.043104E−19   −8.431854E−26   X6Y3−6.345560E−22   2.804678E−22 4.289367E−21 2.349972E−24 −4.743148E−19  5.385876E−25 X4Y5 −1.925796E−22   9.316727E−22 −1.053643E−21  −3.225767E−25   1.860041E−19 1.381096E−24 X2Y7 8.214685E−23 2.388724E−218.375537E−22 2.766796E−25 1.013965E−19 1.617787E−24 Y9 1.703546E−244.526481E−21 −2.966098E−23   1.800745E−26 3.422243E−22 6.810995E−25 X101.991274E−24 5.335545E−26 1.741970E−24 7.205669E−28 0.000000E+00−5.791957E−28   X8Y2 6.491228E−24 1.977752E−24 1.571441E−23 1.716942E−260.000000E+00 −1.179271E−26   X6Y4 4.259954E−25 7.623140E−24−1.086567E−23   0.000000E+00 0.000000E+00 −1.124411E−26   X4Y68.190088E−25 1.642262E−23 −1.531617E−24   0.000000E+00 0.000000E+00−6.908146E−27   X2Y8 −3.305040E−26   2.718356E−23 −1.734683E−24  4.000570E−29 0.000000E+00 −4.575592E−27   Y10 −5.699224E−27  2.657964E−23 5.982496E−26 4.841412E−30 0.000000E+00 −1.211899E−2  Nradius   1.00E+00   1.00E+00   1.00E+00   1.00E+00   1.00E+00  1.00E+00 Coefficient Y-decenter −262.562 −14.529 −294.373 184.266−286.525 −283.6091 X-rotation −5.767 −4.073 2.602 −13.391 0.685 0.041

FIG. 12 shows a projection optical system 49 which can be used in theprojection exposure installation 1 in the case of UV illuminationinstead of the projection optical system 6. Components or referencequantities which correspond to those which have been previouslyexplained with reference to FIGS. 1 to 11 have the same referencenumerals and will not be discussed in detail again.

The projection optical system 49 also has six reflection mirrors whichare denoted with the reference numerals 50 to 55 in accordance withtheir sequence in the imaging light path, from the object plane 4. Themirrors will also be referred to in the following as M1 to M6. In theprojection optical system 49, all of the reflection surfaces are formedas free-form surfaces which cannot be described by a rotationallysymmetrical function.

In the configuration shown in FIG. 12, the sequence of the base curvesof the mirror is the same as in the configuration of FIG. 11. Again, thefirst mirror is only very slightly curved and can thus be simplyconverted into a mirror with a zero base curve (planar base curve) or toa mirror with a convex base curve.

Each of the first three mirrors 50 to 52 has negative angularmagnification of the principal ray. The angular magnification of theprincipal ray of the fourth mirror 53 is infinite since the principalray 26 extends perpendicular to the image plane 8 after reflection onthe fourth mirror 53.

The projection optical system 49 has an image-side numerical aperture of0.7. The projection optical system 49 has an intermediate imagenumerical aperture of 0.14.

In the projection optical system 49, the free working distance d_(w) is20 mm.

The projection optical system 49 has a reduction factor of 8.

In the projection optical system 49, the image field dimensionscorrespond to those of the projection optical systems 6, 35 and 42. Theimage field dimensions are 13×1 mm².

In the projection optical system 49, the maximum angle of reflectionalso occurs in the outer edge rays reflected on the fifth mirror 54 andis α=23.8°. The quotient of the maximum angle of reflection of theimaging light 3 within the projection optical system and the image-sidenumerical aperture is 34°.

The optical data for the projection optical system 49 are againsummarised in the following:

The image-side numerical aperture NA is 0.7. The dimensions of the imagefield 7 are 1×13 mm². The reducing magnification level is 8×. The imagefield 7 is rectangular. The wavelength of the illumination light 3 is193.0 nm. The sequence of the optical effects of the mirrors M1 to M6(negative N; positive P) is PPNPNP. Principal rays enter the projectionoptical system 49 in a convergent manner from the object plane 4. Anaperture stop is arranged on the mirror M2 for limiting the illuminationlight at the edge. The z-distance between the object plane 4 and theimage plane 8 is 1,700 mm. The object-image shift is 549 mm. 11.6% ofthe surfaces illuminated in the pupil planes are obscured. Theprojection optical system 49 has wavefront error (rms) of 0.053 in unitsof the wavelength of the illumination light. The distortion is 400 nm.The image field curvature is 130 nm. The angle of the principal ray onthe central object field point is 6°. The mirror M1 has dimensions (x/y)of 204×184 mm². The mirror M2 has dimensions of 652×271 mm². The mirrorM3 has dimensions of 192×260 mm². The mirror M4 has dimensions of515×347 mm². The mirror M5 has dimensions of 162×153 mm². The mirror M6has dimensions of 643×619 mm². The sequence of the principal ray angleof incidence of the principal ray 26 of the central object field pointon the mirrors M1 to M6 is 5.40°, 8.76°, 11.83°, 5.37°, 0.01° and 0.02°.The sequence of the maximum angle of incidence on the mirrors M1 to M6is 9.70°, 10.06°, 13.22°, 8.94°, 24.01° and 3.62°. The sequence of thebandwidths of the angle of incidence on the mirrors M1 to M6 is 8.23°,2.81°, 3.10°, 6.95°, less than 24.01° and less than 3.62°.

The working distance in the object plane 4 is 200 mm. The workingdistance in the image plane 8 is 20 mm. The ratio of the distancebetween the object plane 4 and the mirror M1 and the distance betweenthe object plane 4 and the mirror M2 is 5.11. The mirrors M1 to M3 havea minimum distance between the used reflection surface and the closestimaging light path which does not act upon the mirrors (free board) ofless than 25 mm. The distance between the object plane 4 and the mirrorM1 and the distances between the pairs of mirrors M1-M2, M2-M3, M3-M4,M4-M5 are greater than 40% of the distance between the object and theimage plane.

The optical design data for the reflection surfaces of the mirrors M1 toM6 can be gathered from the following tables which correspond to thoseof the projection optical system 6 of FIG. 2 described above.

Surface Radius Thickness Mode Object INFINITY 1022.710 Mirror 1−7390.359 −822.710 REFL Mirror 2 513.847 0.000 REFL STOP INFINITY942.710 Mirror 3 501.145 −842.710 REFL Mirror 4 843.206 1380.024 REFLMirror 5 578.181 −417.314 REFL Mirror 6 496.039 437.290 REFL ImageINFINITY 0.000

M1 M2 M3 M4 M5 M6 Coeff. K 3.481687E+02 −9.241869E−01   −7.566344E−01  −5.019615E−01   1.965937E+01 1.267270E−01 Y 0.000000E+00 0.000000E+000.000000E+00 0.000000E+00 0.000000E+00 0.000000E+00 X2 6.555377E−04−5.511453E−04   2.158158E−04 −1.699472E−04   2.894217E−04 5.126962E−06Y2 3.295088E−05 −8.776483E−04   −9.084036E−04   −2.883162E−04  3.472889E−04 1.671956E−05 X2Y −4.245568E−07   3.113324E−08 7.395458E−077.821775E−08 −4.476295E−07   −6.764774E−09   Y3 1.390824E−08−1.918862E−07   8.435308E−08 −2.628080E−08   5.451515E−08−2.659596E−09   X4 −3.307013E−10   −1.191040E−11   3.063977E−09−3.668514E−11   5.377968E−09 8.032524E−12 X2Y2 3.290269E−09−6.921528E−11   1.233667E−11 1.187534E−10 2.249411E−08 2.023497E−11 Y4−1.463471E−10   5.786874E−11 −6.021292E−12   −1.106757E−10  7.037151E−09 −5.631157E−12   X4Y 2.736617E−12 4.032934E−15−6.984058E−13   −8.039415E−14   −1.260298E−12   −5.006977E−15   X2Y33.522297E−13 −1.166725E−13   −2.454747E−12   5.957814E−13−1.250078E−11   −5.698119E−15   Y5 −2.490692E−13   2.590308E−12−3.745572E−13   −1.408338E−14   −2.442407E−11   −7.179108E−15   X6−1.862455E−14   6.281324E−18 −2.148629E−14   −6.004672E−17  1.997946E−13 −1.011352E−17   X4Y2 −7.981936E−14   4.496399E−17−1.242837E−14   −6.611499E−16   2.590470E−13 −6.909855E−17   X2Y4−4.901925E−14   −3.029567E−16   −3.758114E−15   9.515240E−16−2.673556E−13   −1.224111E−16   Y6 2.434885E−16 1.266995E−141.367511E−16 6.466128E−17 2.511816E−13 −4.838450E−17   X6Y 2.013361E−161.162633E−19 1.149857E−17 −3.125791E−19   −1.332065E−15  −1.469592E−20   X4Y3 3.552832E−16 1.010087E−18 −1.441396E−16  −1.842092E−18   −2.995433E−15   −1.117419E−19   X2Y5 −9.924040E−19  −2.022287E−19   −1.400280E−17   1.100935E−18 −2.362122E−15  −1.093754E−19   Y7 1.950700E−18 −1.249257E−17   6.126115E−193.018212E−20 2.029387E−15 −2.279935E−20   X8 −1.816371E−19  8.241847E−23 1.607901E−19 −2.596493E−23   6.322415E−18 −1.205865E−22  X6Y2 −1.231881E−18   1.602604E−21 2.552251E−19 −7.939427E−22  1.136621E−17 −2.391492E−22   X4Y4 −1.457234E−19   1.343999E−20−6.277420E−19   −2.461049E−21   −7.995361E−19   −1.719723E−22   X2Y65.627869E−19 1.086725E−20 2.371593E−20 9.514060E−22 −3.361939E−17  −2.245468E−22   Y8 3.626451E−21 −2.072810E−20   −3.369745E−21  −6.523915E−23   −4.042492E−18   −1.070962E−22   X8Y 1.644403E−215.521298E−25 7.387878E−22 −4.934005E−26   −4.739358E−20   1.327526E−25X6Y3 2.012939E−21 1.839641E−23 6.948031E−22 −5.010250E−25  −3.213699E−19   8.788103E−25 X4Y5 −9.196304E−22   1.613032E−22−7.384331E−22   −1.017620E−24   −4.869993E−19   1.435145E−24 X2Y7−8.444082E−22   4.724249E−22 1.160142E−22 5.807469E−25 −3.565433E−19  5.071171E−25 Y9 −1.391751E−24   −1.535204E−22   −1.540508E−24  3.217510E−27 −5.879640E−20   −1.515906E−26   X10 0.000000E+000.000000E+00 0.000000E+00 0.000000E+00 0.000000E+00 0.000000E+00 X8Y20.000000E+00 0.000000E+00 0.000000E+00 0.000000E+00 0.000000E+000.000000E+00 X6Y4 0.000000E+00 0.000000E+00 0.000000E+00 0.000000E+000.000000E+00 0.000000E+00 X4Y6 0.000000E+00 0.000000E+00 0.000000E+000.000000E+00 0.000000E+00 0.000000E+00 X2Y8 0.000000E+00 0.000000E+000.000000E+00 0.000000E+00 0.000000E+00 0.000000E+00 Y10 0.000000E+000.000000E+00 0.000000E+00 0.000000E+00 0.000000E+00 0.000000E+00 Nradius  1.00E+00  1.00E+00   1.00E+00   1.00E+00   1.00E+00   1.00E+00Coefficient Y-decenter −197.752 −67.646 −76.378 −20.289 −432.652−422.877 X-rotation −1.837 −3.960 −2.990 −9.847 −0.659 −1.856

FIG. 13 shows a microscope lens 56 which can be used for inspectingprojection masks required for projection exposure or lithography or forinspecting of exposed wafers 10. The microscope lens images a microscopeobject plane or substrate plane 57, which coincides with the image plane8 during projection of the projection exposure installation 1, on amicroscope image plane 58. The construction of the microscope lens 56 issimilar, for example, to that of the projection object 6 in FIG. 2, withthe difference that, in the microscope lens 56, the object and imageplanes are exchanged in comparison to the projection optical system 6.The object to be analysed is therefore located at the high apertureportion of the microscope lens 56 and an image-recording device, forexample a CCD camera is located at the low aperture portion of themicroscope lens 56. In the light path between the microscope image plane58 and the substrate plane 57, the microscope lens 56 has a total offour mirrors 59 to 62 which are numbered in this order and are alsoreferred to as M1 to M4. The third mirror 61 and the fourth mirror 62 ofthe microscope lens 56 correspond to the mirrors M5, M6 of thepreviously discussed projection optical systems in terms of their designpositions and the through-holes 23, 24. The four mirrors 59 to 62 areconfigured as free-form surfaces which cannot be described by arotationally symmetrical function. Alternatively, it is also possiblefor at least one of the mirrors 59 to 62 to have a free-form reflectionsurface of this type.

The first mirror 59 has negative angular magnification of the principalray. The second mirror 60 has infinite angular magnification of theprincipal ray, since the principal ray 26 extends perpendicularly to thesubstrate plane 57 from the second mirror 60. The angular magnificationsof the principal ray of the third mirror 61 and the fourth mirror 62 arecorrespondingly undefined.

The microscope lens 56 has a numerical aperture of 0.7. The microscopelens 56 has an intermediate image-side numerical aperture of 0.17.

In the microscope lens 56, the maximum angle of reflection α is againachieved by the outer edge rays of the mirror 57 including thethrough-hole 24 and is 24°. Correspondingly, the quotient of this angleof reflection and the numerical aperture is 34°.

The projection optical systems 6, 35, 42, 49 and the microscope lens 56may be operated using wavelengths of the illumination light 3 other thanEUV wavelengths. For example, it is also possible to use the free-formconstructions for visible wavelengths.

The projection optical systems 6, 35, 42, 49, the microscope lens 56 andthe optical systems described in the following in relation to FIGS. 14to 17 can be constructed in such a way that, with the exception of thelight path in the region of the through-holes 23, 24, there is always adistance of less than 25 mm, but greater than 1 mm (e.g., greater than 5mm) maintained between the individual rays 14 and the respective mirrorM1 to M6 not acted upon, or 59 to 62 when acted upon by reflection ofthe illumination light 3 in the desired manner. This simplifies theconstructional requirements of the respective optical system.

FIG. 14 shows a further configuration of a projection optical system 63which can be used in the projection exposure installation 1, again withEUV illumination, instead of the projection optical system 6. Componentsor reference quantities which correspond to those previously discussedin relation to the projection optical systems 6, 35, 42, 49 of FIG. 1 to12 have the same reference numerals and will not be discussed in detailagain. In the following only the substantial differences between theprojection optical system 63 and the previously explained projectionoptical systems 6, 35, 42, 49 will be discussed.

The optical data for the projection optical system 63 are as follows:

The image-side numerical aperture NA is 0.6. The dimensions of the imagefield 7 are 1×13 mm². The reducing magnification level is 8×. The imagefield 7 is rectangular. The wavelength of the illumination light 3 is13.5 nm. The projection optical system 63 has six mirrors M1 to M6. Thesequence of the optical effects of the mirrors M1 to M6 (negative N;positive P) is NPNPNP. The single intermediate image of the projectionoptical system 63 is present between the mirrors M4 and M5. Principalrays enter the projection optical system 63 in a convergent manner fromthe object plane 4. An aperture stop for limiting the illumination lightat the edge is arranged on mirror M3. The z-distance between the objectplane 4 and the image plane is 1,500 mm. The object-image shift is 7.07mm. 5.7% of the surfaces illuminated in the pupil planes are obscured.The projection optical system 63 has a wavefront error (rms) of 0.034 inunits of the wavelength of the illumination light 3. The distortion is15 nm. The image field curvature is 10 nm. The angle of the principalray at the central object field point is 5.9°. The mirror M1 hasdimensions (x/y) of 126×73 mm². The mirror M2 has dimensions of 339×164mm². The mirror M3 has dimensions of 100×96 mm². The mirror M4 hasdimensions of 196×150 mm². The mirror M5 has dimensions of 307×298 mm².The mirror M6 has dimensions of 814×806 mm². The sequence of theprincipal ray angle of incidence of the principal ray 26 of the centralobject field point on the mirrors M1 to M6 is 18.61°, 8.76°, 15.44°,8.53°, 0.00° and 0.00°. The sequence of the maximum angle of incidenceon the mirrors M1 to M6 is 26.60°, 11.80°, 15.98°, 12.32°, 20.14° and5.11°. The sequence of the bandwidths of the angle of incidence on themirrors M1 to M6 is 16.06°, 6.30°, 1.03°, 7.87°, less than 20.14° andless than 5.11°. The sequence of the angular magnification of theprincipal ray of the mirrors M1 to M3 (negative N; positive P) is NPN.The working distance in the object plane 4 is 102 mm. The workingdistance in the image plane is 40 mm. The ratio of the distance betweenthe object plane 4 and the mirror M1 and the distance between the objectplane 4 and the mirror M2 is 4.13. The mirrors M1 and M4 have a minimumdistance between the used reflection surfaces and the closest imaginglight path which does not act on the mirror (free board) of less than 25mm. The distances between the pairs of mirrors M2-M3, M4-M5, M5-M6 andthe distance between the mirror M6 and the image plane 8 are less than40% of the distance between the object plane 4 and the image plane 8.

The optical design data for the reflection surfaces of the mirrors M1 toM6 of the projection optical system 63 can be gathered from thefollowing tables, which correspond to the tables provided for theprojection optical system 6 in accordance with FIG. 2.

Surface Radius Thickness Mode Object INFINITY 423.049 Mirror 1 291.429−320.693 REFL Mirror 2 682.291 698.472 REFL Mirror 3 327.553 0.000 REFLSTOP INFINITY −250.085 Mirror 4 398.721 909.257 REFL Mirror 5 1753.638−620.641 REFL Mirror 6 834.258 660.641 REFL Image INFINITY 0.000

M1 M2 M3 M4 M5 M6 Coeff. K −9.797768E−01   −2.654407E−01   3.633187E+00−2.607926E−01   3.367484E+01 3.003345E−01 Y 0.000000E+00 0.000000E+000.000000E+00 0.000000E+00 0.000000E+00 0.000000E+00 X2 −6.757907E−04  −9.897313E−05   −6.055737E−04   −1.712326E−04   8.316524E−063.449849E−05 Y2 −6.711750E−04   −1.286106E−04   −5.464279E−04  −1.127817E−04   1.666997E−06 3.303139E−05 X2Y −1.718471E−07  8.106102E−09 3.559721E−08 −1.625547E−07   −3.433987E−08  −5.594447E−10   Y3 8.441316E−08 2.066449E−08 2.993241E−07−2.438542E−07   −5.340235E−09   2.648587E−10 X4 −4.235340E−10  −6.184068E−11   −1.590557E−08   −5.148175E−11   9.293663E−101.431375E−12 X2Y2 −2.833593E−10   −1.232739E−10   −2.294580E−08  6.076202E−11 1.884838E−09 4.501941E−12 Y4 −6.283000E−09  −1.538541E−11   −7.807703E−09   4.592939E−10 9.735975E−10 3.169895E−12X4Y 5.216941E−13 1.355055E−14 −4.125213E−11   −5.236068E−13  −6.108177E−14   4.760532E−16 X2Y3 −5.462082E−12   1.539145E−135.882108E−11 −7.857103E−13   5.606699E−14 −1.383433E−15   Y53.841515E−11 −4.826907E−15   6.536341E−11 −1.173929E−12   6.122980E−14−1.198686E−15   X6 −2.961655E−14   −5.649609E−16   −5.319482E−13  1.037860E−15 1.575126E−15 5.280799E−19 X4Y2 −6.986732E−15   1.523728E−17−1.125923E−12   4.138161E−15 5.066143E−15 3.110524E−18 X2Y4 5.755669E−14−1.992110E−16   −9.962349E−13   −5.642387E−15   5.364157E−153.810873E−18 Y6 −7.476803E−14   −3.652597E−17   −1.721064E−13  −2.311791E−16   1.498586E−15 9.716738E−19 X6Y 8.136042E−16 1.347989E−181.560712E−15 −3.431381E−17   1.006276E−18 −1.255738E−22   X4Y31.102636E−17 9.697709E−22 2.841374E−15 −6.361244E−17   −5.733345E−19  −1.261922E−21   X2Y5 1.331907E−16 −1.331590E−20   2.163234E−152.657780E−17 −1.545019E−18   −3.386914E−22   Y7 3.093492E−170.000000E+00 2.304330E−15 1.049058E−19 3.738255E−20 1.710371E−22 X81.506508E−18 5.810497E−21 1.133674E−17 6.127110E−21 3.186325E−211.107455E−24 X6Y2 −1.013674E−17   6.179938E−22 −5.629342E−17  3.657501E−19 2.411205E−20 2.133982E−24 X4Y4 −1.366007E−18  −3.261229E−22   −8.750490E−17   4.374764E−19 3.931624E−20 4.739463E−24X2Y6 −1.047171E−18   −1.345299E−22   −1.260161E−17   −6.674633E−20  2.052091E−20 3.396921E−24 Y8 −9.482484E−19   −7.567828E−23  −3.447928E−18   −3.054349E−21   6.173346E−21 9.678311E−25 X8Y−5.877725E−20   −1.822355E−23   −4.253705E−19   −1.365311E−22  −1.472429E−23   2.361551E−27 X6Y3 4.790823E−20 −3.116535E−24  −6.154610E−19   −1.894833E−21   −3.675978E−23   1.990878E−27 X4Y58.584886E−21 −9.980946E−26   2.375768E−19 −1.854722E−21  −2.816555E−23   −4.075851E−27   X2Y7 −1.694967E−20   −4.093120E−26  7.589434E−19 −4.379199E−23   −6.563563E−24   −5.800819E−27   Y92.326792E−21 0.000000E+00 1.307119E−19 −2.515286E−23   2.606727E−24−1.858737E−28   X10 1.401272E−22 6.373969E−27 2.615474E−22 2.577682E−254.145747E−26 −1.274796E−31   X8Y2 3.458862E−22 1.154175E−26−7.752079E−21   5.165996E−25 1.524801E−25 −2.154682E−30   X6Y4−6.486950E−23   −8.465791E−29   −1.437881E−20   3.499212E−242.916563E−25 4.867171E−30 X4Y6 −2.005656E−23   −2.584491E−28  −1.352099E−21   3.142335E−24 3.587746E−25 1.828109E−29 X2Y8 6.434247E−23−5.536465E−29   7.452494E−21 3.871445E−25 2.307038E−25 1.576792E−29 Y101.692634E−24 0.000000E+00 1.578385E−21 1.350146E−25 2.372597E−261.664967E−30 Nradius   1.00E+00   1.00E+00   1.00E+00   1.00E+00  1.00E+00   1.00E+00 Coefficient Y-decenter 72.424 −276.725 184.767−26.657 97.145 97.828 X-rotation −3.803 24.855 1.633 24.917 0.012 −0.062

FIG. 15 shows a further configuration of a projection optical system 64which can be used in the projection exposure installation 1, again withEUV illumination, instead of the projection optical system 6. Componentsor reference quantities corresponding to those which have previouslybeen explained with reference to FIG. 1 to 12 or 14 have the samereference numerals and will not be discussed in detail again.

The optical data of the projection optical system 64 are summarised inthe following:

The image-side numerical aperture NA is 0.7. The dimensions of the imagefield 7 are 1×13 mm². The reducing magnification level is 8×. The imagefield 7 is rectangular. The wavelength of the illumination light 7 is13.5 nm. The projection optical system 64 has six mirrors M1 to M6. Thesequence of the optical effects of the mirrors M1 to M6 (negative N;positive P) is NPNPNP. The single intermediate image plane of theprojection optical system 64 is present between the mirrors M4 and M5.Principal rays enter the projection optical system 64 in a convergentmanner from the object plane 4. An aperture stop for limiting theillumination light at the edge is arranged on mirror M3. The z-distancebetween the object plane 4 and the image plane 8 is 1,483 mm. Theobject-image shift is 13.86 mm. 6.4% of the surfaces illuminated in thepupil planes are obscured. The projection optical system 64 has awavefront error (rms) of 0.062 in units of the wavelength of theillumination light 3. The distortion is 18 nm. The image field curvatureis 10 nm. The angle of the principal ray at the central object fieldpoint is 5.9°. The mirror M1 has dimensions (x/y) of 134×84 mm². Themirror M2 has dimensions of 365×174 mm². The mirror M3 has dimensions of121×114 mm². The mirror M4 has dimensions of 220×176 mm². The mirror M5has dimensions of 363×354 mm². The mirror M6 has dimensions of 956×952mm². The sequence of the principal ray angle of incidence of theprincipal ray 26 of the central object field point on the mirrors M1 toM6 is 20.86°, 10.26°, 17.50°, 9.84°, 0.00° and 0.00°. The sequence ofthe maximum angle of incidence on the mirrors M1 to M6 is 29.83°,13.67°, 18.09°, 14.40°, 24.60° and 5.70°. The sequence of the bandwidthsof the angle of incidence on the mirrors M1 to M6 is 18.23°, 7.18°,1.06°, 9.50°, less than 16.98° and less than 5.51°. The sequence of theangular magnification of the principal ray of the mirrors M1 to M3(negative N; positive P) is NPN. The working distance in the objectplane 4 is 100 mm. The working distance in the image plane 8 is 40 mm.The ratio of the distance between the object plane and the mirror M1 andthe distance between the object plane 4 and the mirror M2 is 4.13. Themirrors M1 and M4 have a minimum distance between the used reflectionsurface and the closest imaging light path not acting upon the mirrors(free board) of less than 25 mm. The distances between the pairs ofmirrors M2-M3, M4-M5, M5-M6 and the distance between the mirror M6 andthe image plane 8 are greater than 40% of the distance between theobject plane 4 and the image plane 8.

The optical design data for the reflection surfaces of the mirrors M1 toM6 of the projection optical system 64 can be inferred from thefollowing tables, which correspond to the tables provided for theprojection optical system 6 according to FIG. 2.

Surface Radius Thickness Mode Object INFINITY 413.264 Mirror 1 289.172−313.264 REFL Mirror 2 680.603 689.549 REFL Mirror 3 333.217 0.000 REFLSTOP INFINITY −255.285 Mirror 4 400.498 908.331 REFL Mirror 5 1757.579−620.526 REFL Mirror 6 834.338 660.526 REFL Image INFINITY 0.000

M1 M2 M3 M4 M5 M6 Coeff. K −1.030576E+00   −2.635304E−01   4.190202E+00−2.532242E−01   3.343958E+01 2.989093E−01 Y 0.000000E+00 0.000000E+000.000000E+00 0.000000E+00 0.000000E+00 0.000000E+00 X2 −6.535480E−04  −9.651094E−05   −6.315149E−04   −1.860891E−04   6.210957E−063.467308E−05 Y2 −6.703313E−04   −1.285085E−04   −5.894828E−04  −1.055800E−04   3.848982E−07 3.293719E−05 X2Y −1.109153E−07  9.418989E−09 5.191842E−07 −1.736028E−07   −3.604297E−08  −1.901465E−09   Y3 −1.849968E−07   1.804370E−08 1.052875E−08−3.008104E−07   −1.255871E−08   −7.306681E−10   X4 −3.455652E−10  −6.435672E−11   −1.959503E−08   −1.181975E−10   9.251123E−109.219996E−13 X2Y2 8.907151E−11 −1.169230E−10   −2.854507E−08  3.223161E−11 1.828013E−09 3.292930E−12 Y4 −6.694084E−09  −1.746102E−11   −1.100719E−08   5.508116E−10 9.590508E−10 2.723624E−12X4Y −6.682583E−13   6.169836E−15 −4.579394E−11   −4.554803E−13  −1.075058E−13   −9.398044E−17   X2Y3 −3.764773E−12   1.837427E−138.072483E−13 −1.108837E−12   1.733346E−14 −1.372960E−15   Y53.946729E−11 1.501209E−15 4.522011E−11 −1.761285E−12   5.059303E−14−1.418313E−15   X6 −2.950759E−14   5.555342E−16 −4.772179E−13  2.049340E−16 1.249728E−15 6.302080E−19 X4Y2 −3.981976E−14   7.309283E−17−1.369581E−12   2.599849E−15 4.180701E−15 1.406199E−18 X2Y4 6.662007E−14−1.567936E−16   −1.344358E−12   −6.991042E−15   4.324958E−159.589967E−19 Y6 −6.296271E−14   5.254697E−18 −3.274586E−13  −1.365187E−15   1.317067E−15 4.531531E−19 X6Y 9.572567E−16−4.550481E−18   −2.349696E−17   −2.327425E−17   1.147404E−18−5.815673E−22   X4Y3 1.729544E−15 −5.168321E−21   −6.343836E−16  −6.844084E−17   1.396280E−18 −1.101533E−21   X2Y5 2.003151E−16−1.086056E−20   7.211912E−17 3.651614E−17 2.129037E−19 −6.825077E−22  Y7 −6.259873E−17   0.000000E+00 1.314567E−15 4.966906E−18 4.944608E−20−3.674224E−22   X8 8.514832E−19 5.499001E−21 −1.315946E−17  1.431441E−20 5.935619E−21 2.351396E−25 X6Y2 −1.930952E−17   1.021410E−20−3.809772E−17   2.893679E−19 2.146809E−20 1.941034E−24 X4Y4−2.629657E−17   −5.261250E−22   −4.023107E−17   4.708584E−192.844557E−20 3.285122E−24 X2Y6 −7.113538E−18   −2.063344E−22  −3.710671E−17   −1.202904E−19   1.718587E−20 6.947595E−25 Y8−6.688170E−19   −9.807129E−23   −1.246348E−17   −1.007426E−20  5.947625E−21 5.352899E−25 X8Y −2.167642E−20   −1.475245E−23  −4.375451E−20   −3.593805E−22   −6.272355E−24   −6.386618E−29   X6Y31.577014E−19 −7.541034E−24   1.407216E−21 −1.733010E−21  −1.503182E−23   −2.378905E−27   X4Y5 1.475476E−19 2.828164E−252.164416E−19 −1.819583E−21   −5.558949E−24   −4.818316E−27   X2Y72.386767E−20 2.916090E−26 4.037031E−19 1.506408E−22 1.500592E−23−2.782420E−27   Y9 2.686189E−21 −3.808616E−26   1.365101E−192.759985E−23 9.373049E−24 3.697377E−29 X10 6.880195E−23 4.028878E−27−3.684363E−22   3.684053E−25 8.977447E−27 1.376079E−31 X8Y2 1.028653E−227.179210E−27 −5.946953E−21   1.412893E−24 6.817863E−26 −3.343096E−30  X6Y4 −4.423830E−22   −2.428875E−28   −1.431825E−20   3.370257E−241.794556E−25 −8.790772E−30   X4Y6 −2.798064E−22   1.268239E−28−9.083451E−21   2.674694E−24 2.401259E−25 −2.285964E−30   X2Y8−1.113049E−23   −1.289425E−30   4.131039E−22 −1.824536E−27  1.599496E−25 5.901778E−30 Y10 1.536113E−24 0.000000E+00 9.866128E−229.363641E−28 1.894848E−26 1.501949E−30 Nradius   1.00E+00   1.00E+00  1.00E+00   1.00E+00   1.00E+00   1.00E+00 Coefficient Y- 76.368−281.911 194.003 −24.759 94.122 96.437 decenter X-rotation −6.675 24.3492.204 25.034 −0.109 −0.453

In the following more optical data for two further microscope lenses 65,66 are summarised which, like the microscope lens 56, can be used forinspecting projection masks required for projection exposure orlithography or for inspecting exposed wafers. Both of these furthermicroscope lenses 65, 66 are shown in FIGS. 16 and 17. The basicfour-mirror construction of the two further microscope lens 65, 66corresponds to that of FIG. 13.

Components in these further microscope lenses 65, 66, which correspondto those which have previously been explained in relation to themicroscope lens 56, have the same reference numerals or designations.

The first of the two further microscope lenses 65, 66, the microscopelens 65, shown in FIG. 16, has an object-side numerical aperture of 0.8.The dimensions of the square object field are 0.8×0.8 mm². Theincreasing magnification level is 10×. The wavelength of theillumination light 3 is 193.0 nm. Other illumination light wavelengthsare also possible, for example a visible wavelength or an EUVwavelength. The sequence of the optical effects of the mirrors M1 to M4(negative N; positive P) is NPNP. The single intermediate image islocated between the mirrors M2 and M3 at the location of thethrough-hole 23 in the mirror M4. Principal rays travel out of themicroscope lens 65 in a divergent manner from the microscope image plane58. The z-distance between the substrate plane 57 and the image plane 58is 1,933 mm. The object-image shift is 477 mm. 21.5% of the illuminatedsurfaces in the pupil planes are obscured. The microscope lens 65 has awavefront error (rms) of 0.004 in units of the wavelength of theillumination light 3. The angle of the principal ray at the centralobject field point is 13.8°. The mirror M1 has dimensions (x/y) of219×216 mm². The mirror M2 has dimensions of 520×502 mm². The mirror M3has dimensions of 202×189 mm². The mirror M4 has dimensions of 742×699mm². The sequence of the principal ray angle of incidence of theprincipal ray 26 of the central object field point on the mirrors M1 toM4 is 10.48°, 3.53°, 0.04° and 0.02°. The sequence of the maximum angleof incidence on the mirrors M1 to M4 is 15.70°, 5.58°, 27.79° and 3.19°.The sequence of the bandwidths of the angle of incidence on the mirrorsM1 to M4 is 11.93°, 4.46°, 27.79° and 3.19°. The working distance in themicroscope image plane 58 is 240 mm. The working distance in thesubstrate plane 57 is 40 mm. The ratio of the distance between themicroscope image plane 58 and the mirror M1 and the distance between themicroscope image plane 58 and the mirror M2 is 5.63. The distancebetween the substrate plane 57 and the mirror M1 and the distancesbetween the pairs of mirrors M1-2 and M2-M3 are greater than 40% of thedistance between the substrate plane 57 and the image plane 58.

The optical design data for the reflection surfaces of the mirrors M1 toM4 of the microscope lens 65 can be gathered from the following tables,which correspond to the tables for the previously described projectionoptical systems. In these tables “object” refers to the microscope imageplane 58. “Image” refers to the substrate plane 57.

Surface Radius Thickness Mode Object INFINITY 1350.229 Mirror 1 240.546−1110.493 REFL Mirror 2 435.560 1653.485 REFL Mirror 3 756.829 −422.992REFL Mirror 4 530.970 462.991 REFL Image INFINITY 0.000

Coefficient M1 M2 M3 M4 K −1.387402E+00  −9.186277E−01  2.479623E+011.846234E−01 Y 0.000000E+00 0.000000E+00 0.000000E+00 0.000000E+00 X2−1.972513E−03  −8.152652E−04  4.304599E−04 3.443510E−05 Y2−2.046135E−03  −8.219532E−04  4.280214E−04 3.442623E−05 X2Y 4.924422E−071.043274E−08 1.420911E−07 −1.467857E−09  Y3 3.892760E−07 1.233789E−081.433179E−07 −1.285787E−09  X4 2.843271E−09 −8.849537E−11  5.644150E−095.790124E−12 X2Y2 6.307229E−09 −1.868473E−10  1.095525E−08 1.192799E−11Y4 3.357640E−09 −9.886660E−11  5.323173E−09 6.015673E−12 X4Y3.303637E−13 1.821786E−14 −9.065558E−13  2.636707E−15 X2Y3 4.517153E−133.654773E−14 −1.999032E−12  4.573973E−15 Y5 −1.472281E−14  1.913697E−14−1.039223E−12  1.907361E−15 X6 −1.567647E−14  −2.778349E−17 3.227077E−14 6.941174E−18 X4Y2 −4.271994E−14  −8.658416E−17 9.037002E−14 1.569376E−17 X2Y4 −3.766656E−14  −8.931045E−17 8.435334E−14 1.111043E−17 Y6 −1.062731E−14  −3.096033E−17  2.620546E−142.369368E−18 X6Y 0.000000E+00 0.000000E+00 0.000000E+00 0.000000E+00X4Y3 0.000000E+00 0.000000E+00 0.000000E+00 0.000000E+00 X2Y50.000000E+00 0.000000E+00 0.000000E+00 0.000000E+00 Y7 0.000000E+000.000000E+00 0.000000E+00 0.000000E+00 X8 0.000000E+00 0.000000E+000.000000E+00 0.000000E+00 X6Y2 0.000000E+00 0.000000E+00 0.000000E+000.000000E+00 X4Y4 0.000000E+00 0.000000E+00 0.000000E+00 0.000000E+00X2Y6 0.000000E+00 0.000000E+00 0.000000E+00 0.000000E+00 Y8 0.000000E+000.000000E+00 0.000000E+00 0.000000E+00 X8Y 0.000000E+00 0.000000E+000.000000E+00 0.000000E+00 X6Y3 0.000000E+00 0.000000E+00 0.000000E+000.000000E+00 X4Y5 0.000000E+00 0.000000E+00 0.000000E+00 0.000000E+00X2Y7 0.000000E+00 0.000000E+00 0.000000E+00 0.000000E+00 Y9 0.000000E+000.000000E+00 0.000000E+00 0.000000E+00 X10 0.000000E+00 0.000000E+000.000000E+00 0.000000E+00 X8Y2 0.000000E+00 0.000000E+00 0.000000E+000.000000E+00 X6Y4 0.000000E+00 0.000000E+00 0.000000E+00 0.000000E+00X4Y6 0.000000E+00 0.000000E+00 0.000000E+00 0.000000E+00 X2Y80.000000E+00 0.000000E+00 0.000000E+00 0.000000E+00 Y10 0.000000E+000.000000E+00 0.000000E+00 0.000000E+00 Nradius 1.000000E+00 1.000000E+001.000000E+00 1.000000E+00 Y-decenter −419.012 −607.162 −478.467 −476.646X-rotation −2.721 8.467 0.209 −0.024

The optical data for the second microscope lens 66, which is shown inFIG. 17 and can also be used instead of the microscope lens 56 in FIG.13, are summarised in the following;

The object-side numerical aperture NA is 0.8. The dimensions of thesquare object field are 0.8×0.8 mm². The increasing magnification levelis 40×. The wavelength of the illumination light 3 is 193.0 nm. Otherillumination light wavelengths may also be used, for example visible orEUV wavelengths. The sequence of the optical effects of the mirrors M1to M4 (negative N; positive P) is NPNP. The single intermediate image islocated between the mirrors M2 and M3 in the region of the through-hole23 in the mirror M4. On the image-side, principal rays travel out of themicroscope lens 66 in a divergent manner. The z-distance between thesubstrate plane 57 and the image plane 58 is 2,048 mm. The object-imageshift is 522 mm. 24.6% of the surfaces illuminated in the pupil planesare obscured. The microscope lens 66 has a wavefront error (rms) of0.016 in units of the wavelength of the illumination light 3. The angleof the principal ray at the central object field point is 17.1°. Themirror M1 has dimensions (x/y) of 59×58 mm². The mirror M2 hasdimensions of 222×197 mm². The mirror M3 has dimensions of 180×163 mm².The mirror M4 has dimensions of 736×674 mm². The sequence of theprincipal ray angle of incidence of the principal ray 26 of the centralobject field point to the mirrors M1 to M4 is 12.23°, 3.81°, 0.10° and0.14°. The sequence of the maximum angle of incidence on the mirrors M1to M4 is 18.94°, 5.66°, 24.95° and 2.75°. The sequence of the bandwidthsof the angle of incidence on the mirrors M1 to M4 is 10.17°, 1.81°,24.95° and 2.75°. The working distance in the microscope image plane 58is 996 mm. The working distance in the substrate plane 57 is 40 mm. Theratio of the distance between the microscope image plane 58 and themirror M1 and the distance between the microscope image plane 58 and themirror M2 is 1.46. The distance between the substrate plane 57 and themirror M1 and the distance between the pair of mirrors M2-M3 is lessthan 40% of the distance between the substrate plane 57 and the imageplane 58.

The optical design data for the reflection surfaces of the mirrors M1 toM4 of the microscope lens 66 can be gathered from the following tables,which correspond to the tables for the previously described microscopelens 65.

Surface Radius Thickness Mode Object INFINITY 1458.431 Mirror 1 138.358−462.391 REFL Mirror 2 352.350 1011.807 REFL Mirror 3 521.060 −429.417REFL Mirror 4 523.773 469.415 REFL Image INFINITY 0.000

Coefficient M1 M2 M3 M4 K 2.186021E−01 −8.967130E−01   1.353344E+011.426428E−01 Y 0.000000E+00 0.000000E+00 0.000000E+00 0.000000E+00 X2−2.119566E−03   −6.122040E−04   3.598902E−04 2.150055E−05 Y2−1.870353E−03   −6.339662E−04   4.023778E−04 2.187467E−05 X2Y−2.390768E−05   6.494155E−08 −2.453628E−07   3.235225E−09 Y3−2.981028E−05   5.780210E−08 −6.744637E−08   4.604016E−09 X41.923306E−08 2.795937E−10 2.925492E−09 −1.313710E−12   X2Y2 4.121148E−075.095698E−10 1.819466E−09 −5.092789E−12   Y4 4.757534E−07 2.387275E−10−6.547683E−10   −2.809211E−12   X4Y −1.446899E−09   5.301791E−13−9.735433E−12   −3.703196E−15   X2Y3 −7.970490E−09   8.235778E−13−4.591548E−11   −1.311139E−14   Y5 −6.911626E−09   5.427574E−13−3.434264E−11   −8.056144E−15   X6 −6.957804E−12   4.031055E−164.869018E−14 −2.032419E−18   X4Y2 −6.520224E−12   3.388642E−151.730353E−13 −5.277652E−18   X2Y4 5.785767E−11 4.106532E−15 8.768509E−14−5.976002E−18   Y6 5.002226E−11 −2.665419E−15   −1.533312E−14  3.256782E−19 X6Y 3.978450E−14 −3.458637E−18   2.808257E−16−2.974086E−21   X4Y3 1.060921E−15 −5.135846E−18   1.062927E−15−1.985462E−20   X2Y5 −2.907745E−13   1.367522E−17 1.248668E−15−1.673351E−20   Y7 −1.895272E−13   2.948597E−17 4.722358E−16−2.273773E−22   X8 0.000000E+00 9.742461E−21 −1.224565E−19  −5.498909E−24   X6Y2 0.000000E+00 −1.149790E−22   −8.469691E−19  −3.121995E−23   X4Y4 0.000000E+00 −3.605842E−20   −9.612391E−19  −9.354588E−23   X2Y6 0.000000E+00 −8.956173E−20   3.862422E−20−1.029099E−22   Y8 0.000000E+00 −6.962503E−20   1.096441E−19−3.729022E−23   X8Y 0.000000E+00 0.000000E+00 0.000000E+00 0.000000E+00X6Y3 0.000000E+00 0.000000E+00 0.000000E+00 0.000000E+00 X4Y50.000000E+00 0.000000E+00 0.000000E+00 0.000000E+00 X2Y7 0.000000E+000.000000E+00 0.000000E+00 0.000000E+00 Y9 0.000000E+00 0.000000E+000.000000E+00 0.000000E+00 X10 0.000000E+00 0.000000E+00 0.000000E+000.000000E+00 X8Y2 0.000000E+00 0.000000E+00 0.000000E+00 0.000000E+00X6Y4 0.000000E+00 0.000000E+00 0.000000E+00 0.000000E+00 X4Y60.000000E+00 0.000000E+00 0.000000E+00 0.000000E+00 X2Y8 0.000000E+000.000000E+00 0.000000E+00 0.000000E+00 Y10 0.000000E+00 0.000000E+000.000000E+00 0.000000E+00 Nradius 1.000000E+00 1.000000E+00 0.000000E+000.000000E+00 Y-decenter −473.594 −625.447 −517.418 −517.782 X-rotation−2.590 13.500 −1.408 −0.608

Other embodiments are in the claims.

1. An imaging optical system, comprising: a plurality of mirrorsconfigured to image an object field lying in an object plane to an imagefield lying in an image plane, at least one of the mirrors having athrough-hole configured so that imaging light can pass therethrough,wherein a reflection surface of at least one mirror is in the form of afree-form surface that lacks any axis of rotational symmetry, and theplurality of mirrors comprises at least four mirrors.
 2. An imagingoptical system according to claim 1, wherein the image plane is arrangedparallel to the object plane.
 3. An imaging optical system according toclaim 1, wherein the imaging light is reflected by the mirrors with amaximum angle of reflection of 25°.
 4. An imaging optical systemaccording to claim 1, wherein the imaging light is reflected by themirrors with a maximum angle of reflection of 20°.
 5. An imaging opticalsystem according to claim 1, wherein the imaging light is reflected bythe mirrors with a maximum angle of reflection of 16°.
 6. An imagingoptical system according to claim 3, wherein a quotient of a maximumangle of reflection of the imaging light within the imaging opticalsystem and a numerical aperture thereof on the image side is at most40°.
 7. An imaging optical system according to claim 1, wherein a mirrorof the plurality of mirrors, which is arranged before a last mirror ofthe plurality of mirrors in an imaging light path in the region of apupil plane, has a convex basic shape.
 8. An imaging optical systemaccording to claim 1, wherein the plurality of mirrors consists of sixmirrors.
 9. An imaging optical system according to claim 1, wherein atleast two of the plurality of mirrors have negative angularmagnification of the principal ray.
 10. An imaging optical systemaccording to claim 9, wherein a mirror of the plurality of mirrors withpositive angular magnification of the principal ray is arranged betweentwo mirrors of the plurality of mirrors with negative angularmagnification of the principal ray.
 11. An imaging optical systemaccording to claim 1, wherein a central imaging beam, directed through alast mirror of the plurality of mirrors and centrally through a pupil,of a central object point encloses an angle of greater than 85° relativeto the image plane.
 12. An imaging optical system according to claim 1,wherein a path of the imaging light directed through the last mirror hasan intermediate image in an intermediate image plane in the region ofthe through-hole in the mirror, a portion of the optical system betweenthe object plane and the intermediate image plane having a reducingmagnification level of at least 2×.
 13. An imaging optical systemaccording to claim 1, wherein a mirror of the plurality of mirrors,which is arranged so as to be a penultimate mirror in a path of theimaging light, and from which the imaging light is reflected to the lastmirror of the plurality of mirrors, has a through-hole for the imaginglight to pass through, the image field being arranged behind thepenultimate mirror so as to be off-centre by not more than a fifth of adiameter of the penultimate mirror.
 14. An imaging optical systemaccording to claim 1, wherein a mirror of the plurality of mirrors,which is arranged so as to be a penultimate mirror in a path of theimaging light, and from which the imaging light is reflected to the lastmirror of the plurality of mirrors, has a through-hole configured toallow imaging light to pass therethrough, the image field being arrangedbehind the penultimate mirror so that the image field is central,relative to the penultimate mirror.
 15. An imaging optical systemaccording to claim 13, wherein the penultimate mirror in the path of theimaging light has a radius of curvature greater than 500 mm.
 16. Animaging optical system according to claim 13, wherein the penultimatemirror in the path of the imaging light has a radius of curvaturegreater than 1,000 mm.
 17. An imaging optical system according to claim13, wherein the penultimate mirror in the path of the imaging light hasa radius of curvature greater than 1,500 mm.
 18. An imaging opticalsystem according to claim 1, wherein during use the imaging opticalsystem illuminates an image field greater than 1 mm².
 19. An imagingoptical system according to claim 1, wherein the imaging optical systemhas a numerical aperture on the image side of at least 0.4.
 20. Animaging optical system according to claim 1, wherein the imaging opticalsystem has a numerical aperture on the image side of at least 0.45. 21.An imaging optical system according to claim 1, wherein the imagingoptical system has a numerical aperture on the image side of at least0.5.
 22. An imaging optical system according to claim 1, wherein theimaging optical system has a numerical aperture on the image side of atleast 0.55.
 23. An imaging optical system according to claim 1, whereinthe imaging optical system has a numerical aperture on an image side ofat least 0.6.
 24. An imaging optical system according to claim 1,wherein the imaging optical system has a numerical aperture on an imageside of at least 0.65.
 25. An imaging optical system according to claim1, wherein the imaging optical system has a numerical aperture on animage side of at least 0.7.
 26. An imaging optical system according toclaim 1, wherein the imaging optical system is telecentric on an imageside.
 27. An imaging optical system according to claim 1, wherein theimaging optical system has an object-image shift of less than 100 mm.28. An imaging optical system according to claim 1, wherein the imagingoptical system has an object-image shift of less than 10 mm.
 29. Animaging optical system according to claim 1, wherein the imaging opticalsystem has an object-image shift of less than 1 mm.
 30. An imagingoptical system according to claim 1, wherein: the object plane and theimage plane are disposed a first distance from one another, theplurality of mirrors includes at least one pair of adjacent mirrorsspaced a second distance from one another, and the second distance ismore than 40% of the first distance.
 31. An imaging optical systemaccording to claim 1, wherein: the plurality of mirrors includes first,second and third mirrors; and during use, a portion of an imaging lightpath between the first mirror and the second mirror is less than 25 mmfrom a reflection surface of the third mirror.
 32. An imaging opticalsystem according to claim 1, wherein during use the imaging light isreflected to the image field by a last mirror in a path of the imaginglight, the last mirror having a through-hole configured so that imaginglight can pass through the through-hole.
 33. A projection exposureinstallation, comprising: an imaging optical system comprising aplurality of mirrors configured to image an object field lying in anobject plane to an image field lying in an image plane, at least one ofthe mirrors having a through-hole configured so that m passtherethrough; and a lens system configured to direct illumination lightto the object field of the imaging optical system, wherein: a reflectionsurface of at least one mirror is in the form of a free-form surfacethat lacks any axis of rotational symmetry; the plurality of mirrorscomprises at least four minors; and the projection exposure installationis configured to be used in microlithography.
 34. A projection exposureinstallation according to claim 33, further comprising a light sourceconfigured to generate illumination light having a wavelength of between10 and 30 nm.
 35. A method, comprising: providing a reticle and a wafer,and using a projection exposure installation to project a structure onthe reticle onto a light-sensitive layer of the wafer to produce amicrostructure on the wafer, wherein: a) the projection exposureinstallation comprises: an imaging optical system comprising a pluralityof mirrors configured to image an object field lying in an object planeto an image field lying in an image plane, at least one of the mirrorshaving a through-hole configured so that imaging light can passtherethrough; and a lens system configured to direct illumination lightto the object field of the imaging optical system; b) a reflectionsurface of at least one mirror is in the form of a free-form surfacethat lacks any axis of rotational symmetry; and c) the plurality ofmirrors comprises at least four mirrors.
 36. A system, comprising: aplurality of mirrors configured to image an image field lying in animage plane to an object field lying in an object plane, at least one ofthe mirrors having a through-hole configured so that imaging light canpass therethrough, wherein a reflection surface of at least one mirroris in the form of a free-form surface that lacks any axis of rotationalsymmetry, the plurality of mirrors comprises at least four mirrors, andthe system is a microscope lens.